BCHydro backarc #22

Implemented BCHydro backarc term, added tests (test results tentatively accepted based on visual comparison to other BCHydro flavors). Also corrected order of z1p0 and z2p5 in header for interface-inputs.csv and slab-inputs.csv (results are not impacted as these fields are NaN for all test configurations). Note that the line endings in these files are only CR so diff treats the files as single line.