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
Multiplication/division of TimeDelta, updating documentation and tests.
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
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
Merge remote-tracking branch 'upstream/master' into new_models_#980_and_#981
Changes in Paramteric1DModel class and minor changes related to deriv()