Fixed Gaussian1dModel stddev
Removed redundant parentheses
Fixed some of the Gaussian1DModel tests
Fixed a couple of tests using Gaussian1DModel
Tidied up and fixed equation for Gaussian2DModel
Fixed expression for derivatives
Reduce noise to make tests of non-linear fitters more robust
Fixed order of parameters in Gaussian2DModel.eval
Swap order of arguments for fwhm and stddev, and do more error checking for input paramters
Moved theta to be 6th argument for Gaussian2DModel
Simplified Gaussian2DModel test
Merge pull request #1106 from astrofrog/modeling/fix-gaussian-stddev

Gaussian1dModel.stddev is incorrectly used

This also fixed points 1 and 2 in #1077