add gruId to output files that contain basin(gru) variables and possibly split basin variables into their own output file with just gru & time dimension..

[andywood]

Two suggestions: 1) If basin gru variable is requested, write the gruId in the output file to facilitate post-processing. 2) if basin gru variable is requested, write a separate (non-hru) file with the gru/gruId and gru variables.

This will particularly facilitate linking output to routing and basin water balances.

happy to tackle this if it's an approved behavior, when there's time

[andywood]

The first half of this is partly done -- a gruId now ends up in the output files but I haven't yet made it conditional on the existence of gru variables in the file.