typing of external libraries

[fzimmermann89]

Some of our dependencies do not direclty include type hints. We currently ignore them in mypy.
We could install the type stubs (scipy, h5py, pydicom) if available.
We those not available (ismrmrd, pypulseq), we could generate our own stubs (using genstubs and some manual fixing) and put them in a stubs directory.
This would allow mypy to check if our code uses these functions correctly. currently, mypy assumes everything from these libraries to be Any and does not check anything.

[fzimmermann89]

..this would also require some minor changes to our code to fix some of our typing errors currently hidden