Clean up psy package

[k1o0]

The +psy package is a little messy at the moment. Documentation and tests would be appreciated.

• [ ] Add docstring for all +psy functions
• [ ] Add a Contents file to the package
• [ ] Add a README with grouping of functions (i.e. which fit functions go with which plot function)
• [ ] Complete the psychofit toolbox, currently some files are missing, perhaps make subpackage?
• [ ] Add tests for all functions
• [ ] Merge with cortexlab +psy
• [ ] Add colormap_pickgreyscale to dependencies
• [ ] #222 in plot2AFUC