Library loading

[suuppl]

Hi,
thank you for this library, it's really easy to use.

I did some minor refactoring and added a feature:

• move the library loading code to the base class and added support to pass in a custom library path (at least for windows)
  • cleans up codebase
  • I had some problems with find_library(), so i added the feature to override the dll path
• change the default values in the base class to None (mostly untested as I don't have any picoscope where I am at the moment)
  • makes it obvious if you've forgotten a value in a scope specific implementation
• update makedocs.bat to call pydoc as a script
  • no dependency on python location
• update the html docs (I just ran makedocs.bat, so no idea if the docs look like they should)

Feel free to cherry-pick what you like, given that I did basically no testing...
Let me know if you have any questions ;)

[pep8speaks]

Hello @suuppl! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file picoscope/picobase.py:

Line 77:19: E261 at least two spaces before inline comment
Line 80:21: E261 at least two spaces before inline comment
Line 80:29: W291 trailing whitespace
Line 81:21: E261 at least two spaces before inline comment
Line 84:25: E261 at least two spaces before inline comment
Line 85:25: E261 at least two spaces before inline comment
Line 86:27: E261 at least two spaces before inline comment
Line 105:24: E261 at least two spaces before inline comment
Line 106:20: E261 at least two spaces before inline comment
Line 108:29: E261 at least two spaces before inline comment
Line 110:21: E261 at least two spaces before inline comment
Line 126:80: E501 line too long (89 > 79 characters)
Line 172:5: E303 too many blank lines (2)
Line 172:80: E501 line too long (83 > 79 characters)
Line 176:80: E501 line too long (118 > 79 characters)
Line 177:80: E501 line too long (132 > 79 characters)
Line 219:80: E501 line too long (85 > 79 characters)
Line 226:80: E501 line too long (114 > 79 characters)
Line 336:80: E501 line too long (120 > 79 characters)
Line 367:80: E501 line too long (119 > 79 characters)
Line 379:80: E501 line too long (90 > 79 characters)
Line 419:80: E501 line too long (93 > 79 characters)
Line 446:80: E501 line too long (84 > 79 characters)
Line 502:80: E501 line too long (114 > 79 characters)
Line 539:80: E501 line too long (90 > 79 characters)
Line 549:80: E501 line too long (88 > 79 characters)
Line 561:80: E501 line too long (101 > 79 characters)
Line 606:80: E501 line too long (109 > 79 characters)
Line 693:80: E501 line too long (99 > 79 characters)
Line 711:80: E501 line too long (117 > 79 characters)
Line 758:80: E501 line too long (89 > 79 characters)
Line 761:80: E501 line too long (99 > 79 characters)
Line 763:80: E501 line too long (80 > 79 characters)
Line 766:80: E501 line too long (81 > 79 characters)
Line 778:80: E501 line too long (118 > 79 characters)
Line 821:80: E501 line too long (98 > 79 characters)
Line 826:80: E501 line too long (86 > 79 characters)
Line 827:80: E501 line too long (84 > 79 characters)
Line 829:80: E501 line too long (114 > 79 characters)
Line 951:80: E501 line too long (101 > 79 characters)
Line 1099:80: E501 line too long (101 > 79 characters)
Line 1129:80: E501 line too long (119 > 79 characters)
Line 1142:80: E501 line too long (114 > 79 characters)
Line 1221:80: E501 line too long (101 > 79 characters)

• In the file picoscope/ps2000.py:

Line 134:1: W293 blank line contains whitespace

• In the file picoscope/ps2000a.py:

Line 128:1: W293 blank line contains whitespace

• In the file picoscope/ps3000.py:

Line 121:1: W293 blank line contains whitespace

• In the file picoscope/ps3000a.py:

Line 135:1: W293 blank line contains whitespace

• In the file picoscope/ps4000.py:

Line 129:1: W293 blank line contains whitespace

• In the file picoscope/ps4000a.py:

Line 190:1: W293 blank line contains whitespace

• In the file picoscope/ps5000.py:

Line 137:1: W293 blank line contains whitespace

• In the file picoscope/ps5000a.py:

Line 143:1: W293 blank line contains whitespace

• In the file picoscope/ps6000.py:

Line 138:1: W293 blank line contains whitespace

• In the file picoscope/ps6000a.py:

Line 213:80: E501 line too long (84 > 79 characters)
Line 215:1: W293 blank line contains whitespace