Make a helper module to consolidate VAF interpreter fields

There was a lot of convoluted code between the ManySamplesVafInterpreter, the basic VafInterpreter, the FullReporter, and the DocmReporter in order to determine the available_fields and field_descriptions. The This PR is an attempt to things clean up by pulling all of the related functions into a helper class.
Would it be better if this was either:

```perl
my %available_fields = basic_field_descriptions('');
return keys %available_fields;
```
or
```perl
return single_vaf_fields(), per_library_vaf_fields();
```
instead of duplicating these names again?