Pipeline fixes and improvements

[mkorbel1]

Description & Motivation

The Pipeline abstraction has some bugs and untested behavior, so this PR addresses those:

• Add a new exception for when a port does not exist, and throw it if get is called on a pipeline for an un-piped signal.
• Mark some SSA signals as mergeable to make generated SystemVerilog prettier and less verbose.
• Expose stageCount for Pipeline
• Improve documentation comments for pipelining.
• Fix a bug where get on pipe stages which did not modify a signal returns a floating signal.
• Add a bunch of new tests for pipelining.

Related Issue(s)

N/A

Testing

Existing tests + new tests added

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