Consider improving how FinancialAssistance payments are described

[philbarker]

Currently we describe FinancialAssistance payments by providing only the total value using a schema:QuantitativeValue:

This is less expressive and more convoluted than how we describe PayProfiles for Jobs. For example:

• we cannot specify whether the FinancialAssistance is a lump sum, per-annum, whatever...
• we cannot specify how often payments are made
• the use of unitText specified as the codedNotation of an Alignment object is much less direct than using the ceterms:currency property directly.

On the other hand there are 1,500 out of 20,000 Financial Assistance Profiles with values, e.g. https://credentialfinder.org/Credential/ce-4af6ea4c-1516-6f2b-2cb6-4a0092d5b692 so we need to avoid breaking changes.