Fix net_to_block_ids method include_global=False

Fix `VprNetParser::net_to_block_ids` method for the case where
`include_global=False`.  Prior to this commit, the behaviour was
equivalent when `include_global` was set to either `True` or `False`,
and included processing of global nets.
Templatize VprNetParser for index-type, cache C...

Templatize `VprNetParser` on the type to use for net/block indexes.
This allows different integer types to be used, which could be helpful
when optimizing memory usage by, e.g., using unsigned-integer types to
double the range of indexes that can be stored, possibly allowing the
use of a smaller unsigned-integer type.

Cache several Cython `cVprNetFileParser` property values to improve
property look-up performance.

Add docstrings to several `cVprNetFileParser` methods.
[NB] Reorganize files into initial Python package