gradient check CLI: allow specifying parameter index/ID

[dweindl]

For large problems, it would be convenient if specific parameters could be selected for gradient check via the command line interface.

• Update gradient checker to accept a list of parameter IDs
• Add CLI option / update arg parsing
• Pass list of IDs through to gradient checker
• Update GradientFunction to allow specifying parameter indices w.r.t. which gradient is to be computed
• Set AMICI plist accordingly