switch oifits int8 columns to bool in schemas and disallow int8 in schemas #766

As came up during discussion of #765 astropy does not support saving int8 to FITS and instead rigidly forces the values to be T F or \x00 (currently in dev astropy, targeting 8.0.1). We already do some coercion of int8 values to bool to comply with previous astropy/FITS limitation.

It may make sense to remove int8 from our schemas (since we can't support it using astropy and FITS):

(just one example), replace it with bool, and disallow int8 in schemas.