Implement an IPAC table writer

This is based on discussions in #833 and and #1143.
@taldcroft
@keflavich
Please use this pull request to suggest your changes.

closes #833
fix itendation errors in documentation
modified for the suggetions on the pull request
improve formatting as the taldcroft suggestion
fixes a small bug that prevented changes in the comment symbol on writing

closes #790
Multiplication/division of TimeDelta, updating documentation and tests.
Comparisons of Time and TimeDelta
implement the changes dicussed on the issue tracker in github
return the identation as much as possible to @taldcroft original design
Add __truediv__ for python 3, as well as one-line doc-strings
Allow __abs__ on array, add test, write div in terms of mul: * 1./other
Add tests, if only to avoid the silly bug of the previous commit
Small documentation update of rounding issues with time arithmetic
Merge pull request #1082 from mhvk/timedelta-multiples

Multiplication/division of TimeDelta
Merge pull request #1152 from hamogu/follow833

Implement an IPAC table writer
Added inverse to some models and SCompositeModel.
Make it easier to subclass Table

Before this patch, every subclass needed to implemented its own
_new_from_slice.
add more tests, replace other hardwired Table
Removed  flag and clarified error message
Merge pull request #1199 from nden/add-inverse

Added inverse to some models and SCompositeModel.