Made Record implement IsRecord
Created PartialEvaluator for expressions to efficiently page over long lists of resources

PartialEvaluator evaluates an expr statically to produce an instance of FieldReader which
will compute a field value directly from a Record.

The PartialEvaluator can be used in any context where indicator definitions are not being changed.
Implemented ExportFormTask and ExportFormExecutor
Merge branch 'master' of github.com:bedatadriven/activityinfo
Fixed regression in TableCalcTest