Change the returned variables to separate sigma level and sigma layer depths
Add code to try to parse the time attribute such that a Modified Julian Day time series for the POLCOMS data can be calcualted. Also means slightly changing the format of the pc struct so that each variables has a data and units array, the latter being used to find the reference time for the seconds in pc.time.data
Allow the input temperature and salinity values to be either single values (for uniform grids) or otherwise 3D arrays of unstructured grid points with sigma layers and time as the two other dimensions. This should maintain backwards compatibilty with the previous invocation