Added inverse to some models and SCompositeModel.
Removed  flag and clarified error message
Merge pull request #1199 from nden/add-inverse

Added inverse to some models and SCompositeModel.
Added new Custom1DModel class and defined new models (not implemented yet)
Fixed some typos in the docs
Changes in Paramteric1DModel class and minor changes related to deriv()
Fixing minor bugs and typos
Added **kwargs to Parametric1DModel
Added col_deriv=1 to NonLinearLSQFitter
Changed Gaussian1D and PowerLaw1D to Parametric1DModel
Added new models
Added new models
Removed commented out code
Moved Gaussian1DModel back to the beginning
Rebased Branch