Upgrade VcdParser to handle more VCD files

[mkorbel1]

Description & Motivation

While ROHD generates relatively simple VCD files, other tools (like Icarus Verilog) take advantage of more features of VCD files. This PR upgrades the VcdParser utility in ROHD so that it can check a wider variety of VCD files. It's still pretty limited, but this helps.

Related Issue(s)

N/A

Testing

Added a new test on a VCD file generated by Icarus Verilog

Backwards-compatibility

Is this a breaking change that will not be backwards-compatible? If yes, how so?

No

Documentation

Does the change require any updates to documentation? If so, where? Are they included?

No