IO library fixes & improvements

[FatsieFS]

This PR contains critical fix for the IOPadVdd cell. Previous gds should not be used anymore for new tape-outs.
For the rest some small improvements to the generated files and docs.

[stafverhaegen-chipflow]

@sergeiandreyev @KrzysztofHerman
Current CDL seems out of date. The IOPadVdd cell in the CDL still contains the ESD diodes and not the RC clamp.

[stafverhaegen-chipflow]

BTW, this thus contain fix for #101
There is still a difference between gds I uploaded in that issue and the current gds. The former still contained a DRC with a connection with too big width on metal2.

[FatsieFS]

I only now see that Boris did still find that the bulk is connected to iovdd; so wait with merging until this is clarified.

[sergeiandreyev]

Hello Staf, let's have a sync-up on IO cells with Alexey Balashov, to be all on the same page
I guess we're going out of sync now..

[sergeiandreyev]

@FatsieFS , we just had a meeting with Mabrains team who are developing KLayout LVS for our OpenPDK..
could you please take a look at the slides on IO cells, if all these items are known/fixed/could be waived
https://docs.google.com/presentation/d/19rNEkZFATAg9Ojm-7nIBbsLfwG7sHYNntAm3xROcy3c/edit?usp=sharing
Thank you in advance!

[dnltz]

@FatsieFS IOPadVdd has missing slits on M2. Maximum width without slits is 30u.

[stafverhaegen-chipflow]

@FatsieFS IOPadVdd has missing slits on M2. Maximum width without slits is 30u.

I updated patch with fixed gds. I did fix it but seems I did not properly update the gds file in the patch.
Also rebased the patch on top of latest dev branch.

[dnltz]

@stafverhaegen-chipflow just tried your new GDS and looks good to me!

[KrzysztofHerman]

@dnltz does it solve the problem for you? Shall we merge ?

[dnltz]

@KrzysztofHerman Slt.c is missing in the Open PDK rules but there is no area wider then 30u anymore. Should be fine.

[stafverhaegen-chipflow]

@KrzysztofHerman
I think you can close #101 also then.
There may still be open questions about LVS as discussed in #109 (comment) but from our f2f discussion I understood Alexey would take the next steps there.