Add support for different ascii vic versions

[tbohn]

The tool needs to be able to understand differences among pre-5.0 VIC versions. As per my email exchange with @jhamman, we can assume that the netcdf files will all be 5.0 image mode (if different image drivers require different formats, we can handle that later).

I expect that pre-5.0 VIC formatting will cluster around 4.0.x, 4.1.2, and 4.2/5.0. I'll work my way backwards starting from 4.2/5.0 and try to accommodate as many formatting options as I can.

[tbohn]

As I continue to work on tonic, I'm finding that the input version string is probably unnecessary for converting parameter files; the existing options describing which variables are present will likely cover all potential options for parameter file structure.

Where the input version string may make more of a difference is in the state files, since their structure varied a little in the 4.0* series, although that might also be handled via an option that names the difference more explicitly. While some of the differences were deliberate, others were the result of bugs (state file not saving all of the information that was necessary, or in the wrong order). I am not sure we should spend effort accommodating those differences...