Some pyroms updates

[kate]

I have been making some changes to some codes in pyroms. Notably:

• Use of multiprocessing for creating BC/CLM files in examples/Arctic2.
• Some files for creating a ROMS rivers file from hydrology model output in examples/rivers. This includes a conversion of the venerable C++ maskedge program to Python.
• A way to merge two or more ocean.in files into one for grid nesting in examples/nesting.