rename Gaussian models and parameters
update tests to reflect name change
rename paramdim to param_dim
rename parnames to param_names
rename psets to param_sets
remove ratio, added y_fwhm to gauss2d
Added cov_matrix option to Gaussian2D
typo in cov matrix
Add test to make a 2d gaussian with a covariance matrix
change mu to mean, sigma to stddev
add x_fwhm, y_fwhm and cov_matrix to docstring
more detail about cov matrix
rename psets to param_sets in docs
rename parnames to param_names in docs
rename paramdim to param_dim in docs
rename gauss1d to gaussian1d in docs
rename xcen to mean in docs
rename xsigma to stddev in docs
update fitting tests
still updating tests to use new kwarg names