[Enhancement]: Add optional valuation_id variable to equity compensation issuances

[arthur-clara]

Description of Enhancement :

Add optional valuation_id variable to equity compensation issuances. This variable would reference the id of the valuation object in the Valuations file.

Why is this Needed?

This is needed to reduce ambiguity about the FMV of an option grant for calculating NSO / ISO

Anything else we need to know?

See conversation on NSO / ISO splits in issue #504

[MattCantor]

We should also add a natural language rule that if a valuation_id isn't provided, then we assume for purposes of the ISO NSO Calculator that the fair market value as of the grant date is equal to the exercise price.

See code and README