Use unique index for connector pin ports

[formatc1702]

Closes #160.

Use unique numerical index instead of string (which might cause issues when using pin names that are not unique when ignoring upper/lower case).

This does not address the issue of having special HTML characters as part of pin names, labels, etc. as mentioned in #160 and now listed as #230; there should be a separate PR dealing with escaping these entities in the GraphViz HTML (e.g. > to >).

[kvid]

I like your implementation with a few simple changes to obtain unique port numbers, but I also suggest using one-based port numbers to keep them identical to the pin numbers in the typical case as you describe in #160 (comment). Adding +1 at four locations is enough to ensure this.

If you agree to this, it might also be wise to name the variables pinindex (or pinidx if it must be short) instead of pinid when they contain zero-based pin index numbers. What do you think?

I also suggest updating the type hints of the Cable.connect() parameters and the Connection attributes to reflect their new pin index usage:

PinIndex = int  # Zero-based pin index
NoneOrMorePinIndices = Union[PinIndex, Tuple[PinIndex, ...], None]  # None, one, or a tuple of zero-based pin indices

[formatc1702]

I have implemented your suggestions, please have a look at the updated code.

[kvid]

My change suggestions have been implemented, and the test cases mentioned in #160 seems to be handled as expected now. When building examples, the ex08.gv and ex09.gv files are then only different files because their internal port lables are numbered differently. However, all PNG files are still equal, as expected.