Support FSDB & SHM Waveforms

[harrisonliew]

ucb-bar/chipyard#1072 and ucb-bar/chipyard#1102 added FSDB support in Chipyard because newer VCS versions now require FSDB output.

This PR selects the waveform format based on the extension. Now, the supported formats are VCD/VPD, FSDB, and SHM (default output of Xcelium).

Includes small bugfixes for PG view generation.

[harrisonliew]

Just learned from @kenhoberkeley that Xcelium prefers SHM output, which is a directory, so this method will not work. Will probably need to emit a key from the sim tool for the output waveform format.

[harrisonliew]

Edit: SHM as default from Xcelium produces .shm in the folder name, so this is fine for now. It assumed that the user would not delete these extensions.

[harrisonliew]

Based on testing, also found out that Voltus reads FSDBs via it's internally-included copy of Verdi. Unfortunately, the Verdi version is a few years old (P-2019.06) even in the latest Voltus 21.12 so it is unable to read FSDBs from later VCS versions that made this switch to FSDB anyway.