Add slice functionality as an IO option

Provide the ability to take slices of the total domain at a user specified location; x, y, z, and at a frequency independent of the standard IO frequency.

This requires:

1. New variables xslice, yslice, zslice added to the .ini file.
2. Deduce the numerical indices and mpi ranks that correspond to the slices in the xy, yz, xz planes.
3. Subcommunicator.
4. Extract to a local buffer and change to single precision.
5. Write to file using HDF5 parallel.