UnsupportedOperationException in IDMapperStack.freeAttributeSearch()

[jgao]

Bridge-rdb and bridge-rest AttributeMappers? support free search, but biomart AttributeMapper? does not. The problem is that if an IDMapperStack contains both IDMapperRdb and IDMapperBiomart, IDMapperStack.freeAttributeSearch() will throw an UnsupportedOperationException?. To solve this problem, we could add another method isFreeAttributeSearchSupported() in AttributeMapper?, similar as in IDMapper. Or we can wrap getAttributeSet() and isFreeAttributeSearchSupported() in AttributeMapperCapacities?, just like what we did for IDMapper.

This problem can be reproduce in CyThesaurus? by stacking an IDMapperBiomart and IDMapperRdb and mapping from Symbol to any accessions.