Changing FITS table write function to have default value of Nan for float type cols
Changing default value for float columns from 1e20 to Nan in fits table write function
Changing the float comparison for filled value to np.allclose to enable Nan masking for float32 as well. Also added test cases for float64, and float32
Removing unnecessary comment
Added changelog for bug fix
Merge pull request #2196 from shaileshahuja/default-fill-Nan-table-writer(Issue#2186)

Default fill nan table writer(issue#2186)