updated / added xschemrc for better useability for non IIC-OSIC-Tools user

[simi1505]

On the weekend, I played with the xschemrc files, which are really helpful. If used correctly no XSCHEM_USER_LIBRARY_PATH in the .designinit is needed anymore. Just navigate in the xschem folder and open xschem and all paths are set automatically. Therefore, this course can be used without IIC-OSIC-Tools even faster.
I also added one xschemrc file to the cace/templates folder and updated the absolute paths in the template testbenches to relative ones. So, now everyone can open the testbenches, symbols, and schematics with Xschem, and all relevant subcircuits are referenced correctly. No new referencing of the symbols is needed.

By the way, this is useful for bigger projects with various IPs. There, the xschemrc files are called recursively and every IP is loaded into Xschem with the correct path.
Template: https://github.com/efabless/sky130_ef_ip__template

I also found out that the CACE template testbenches noise & tran of the improved-OTA still included the ota-5t. I changed the symbols accordingly. However, the improved-ota has some issues during transient analysis when VDD, VIN and Temperature are sweept. The simulation fails with "Timestep too small". Therefore, for now, only the corners are swept in the tran testbench.
The improved-ota also fails the noise limits. Therefore, for now, I did NOT change the CACE output files (.md and figures). This is something we should have a look at in the future.

[hpretl]

@simi1505 Thanks for this update, I think that indeed this makes the experience smoother. I have not played much with xschemrc, probably a mistake. I do not merge for now, I need to look first into the issue you found with improved OTA, thanks for finding it. Let's work on this PR/branch until all issues are fixed, and then we merge in one swoop.