Added new Custom1DModel class and defined new models (not implemented yet)
Fixed some typos in the docs
New Parametric1DModel and Custom1DModel class
Merge remote-tracking branch 'upstream/master' into new_models_#980_and_#981
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
Fixed typo in ModelDefinitionError