make IPAC table fill_values right on output

In the last implementation, IPAC tables simply reported the fill_value as the
null value in the header, but did not use the same value to fill the masked values
in the table data - `ascii.write(... fill_values=(...))` was used for that.
Thus, on writing the null value reported in the header looks good, but is
useless.

closes #2657
fix whitespace for doctest (pep8)
Rename Scale.factors -> Scale.factor and Shift.offsets -> Shift.offset
Merge pull request #3027 from embray/modeling/scale-shift-parameter-rename

Rename parameters for Shift and Scale models
changed Beta1D and Beta2D to Moffat1D and Moffat2D
added CHANGELOG entry and make some recommended changes with capitalization
Improve entry in CHANGES.rst
Change io.ascii meta 'comment_lines' to 'comments'
Fix test regression
testguide.rst - fix minor typo for sphinx
Merge pull request #3258 from hamogu/patch-1
Merge pull request #3260 from hamogu/rename-beta1d-and-beta2d-issue-3029

Rename beta1d and beta2d - solves issue 3029 (This takes over #3102)
Fix broken links pointed out in https://github.com/astropy/astropy/issues/2895
Merge pull request #3259 from hamogu/fix_2657_simple

make IPAC table fill_values right on output
Minor tweak to exception message [skip ci]
Merge pull request #3256 from taldcroft/ascii-comments

Unify io.ascii meta comments for all cases to the key "comments"
Fixed comment writing system for commented-header
Changed hardcoded newlines into multi-line string
Allow for disabling comments during writing, doc changes
Set write_comment to False for readers with no default write_comment