Tests and inverse function for Anderson's TI needed

[andreww]

I added support to MS_TI for Anderson's 1961 parameterisation in 68ec9df but we don't have test cases, or the inverse function in MS_TI_parameters. We should add these. It is probably time to rework MS_TI_parameters so that the output gets put in varargout rather than dumping a massive list of outputs (we can do this without breaking backwards compatibility).

[andreww]

Also, fix the units in MS_TI documentation (density is per cu. m not per sq. m)