GEOMESA-591 Export to binfile format

britt@ccri.com submitted this for PR...I'll address any comments for him and commit it once it's reviewed.
Instead of using 'dtg' as the default, you can try pulling it out of the simple feature type. See org.locationtech.geomesa.core.index#getDtgFieldName. It returns an Option, so you'd still need the final 'dtg' fallback I guess.