Mid cleanup of FluidVec when writing NAs & mixed int/floats

vec.set_type busted; needs to be atomic w/set_writable for setting type.
Need overrides of set_NA in all chunk subtypes.
More cleanup in FVecTest needed.
Turn on GBM/DRF regression-tree tests
Still have debug prints in DRF & GBM
Massive FVec API cleanup

Define at/at8/isNA/set/setNA clearly, consistently & concisely.
Loading a missing value as a long "at8" throws; loading as a
double returns Double.NaN.
Setting a Double.NaN sets NA also.
Remove vector dtype.
Put the isInt() call in the RollupStats.
Cleanup the RollupStats.
Merge branch 'master' into cliffc-fvec3