Prepare ROPP for L1 & L5

[Ian Culverwell]

A reviewer at the ROPP-11 DRR pointed out that a lot of the coding, documentation and testing of the dual frequency parts of ROPP is framed in terms of L1 and L2 frequencies. For example, the bending angles at the two frequencies are called bangle_L1 and bangle_L2. This is clearly not appropriate in an era when RO satellites will be tracking L5 signals. ROPP-12 is probably a good time to convert code, documentation and test scripts to something like bangle_f1 and bangle_f2 (etc), without any assumption that f1=L1 and f2=L2. (The means of reading channel frequencies from input files and storing them in the ROprof structure for use throughout the code is a wider one, which should probably be addressed in a separate ticket.)