Feature: Add control flow to handle optional parameters for Serpent2 cards

[yardasol]

Background and motivation

Certain Serpent2 cards have optional parameters. For example, the pop card has the following parameters:

set pop NPG NGEN NSKIP [ K0 BTCH NEIG ]

The bracketed values are optional. In the current implementation of SaltProc, a user using these optional parameters on the set pop card will get a ValueError: too many values to unpack on line 90 in serpent_depcode.py

Description of idea

We should add control flow to handle these optional inputs.

Implementation details

We will need to:

1. Identify which cards SaltProc interacts with directly
2. Implement control flow to handle any optional parameters

Potential snags

None