Support statements of expected/acceptable migration outputs into Business Rules

[jackdos]

The specification for a business rule explicitly states lists of formats/format families that the rule can operate on but doesn't say anything about the output. This was for two reasons:

1. business rules can apply to any type of preservation action and only migration types would have outputs whose formats you care about
2. the actual output of running a preservation action may be specific content dependent, i.e. if you have two files of the same format but which use different feature sets of that format, a tool might create outputs of two different formats for the same requested action.

However, in order to validate migrations, we are still likely to want to be able to check something about the output. For example, migrate-to-pdfa might apply to multiple input formats, and we might expect that it can produce PDF/A 1a (fmt/95) or PDF/A 1b (fmt/354), but that if it produces vanilla PDF 1.7, something has failed.

Proposal: we should allow an optional list element of expected/acceptable output formats, and/or an optional list element of expected/acceptable output format families to be specified on business rules.