pst_from support for single column parameter files about pyemu

[briochh]

@wkitlasten this seems like something that should be supported. I would vote for using index_col=None but specifying use_col=0 for the headerless single column list file type.

The main thing to look out for is that (I think) the presence of a non None index_col is what triggers a lot of assumptions about the input file structure (ie list rather than array style). This could be switched over to use_cols being the trigger and the index that is required for the parameter name being derived from the dataframe index.

There maybe a bit of chasing it through required. Give a shout if you get stuck.

[briochh]

The option here is to support single col inputs as either list-like or array-like.
For list-like, we would need to support passing to add_parameters() without index_cols argument and move the list-like setup trigger to use_cols. May just need chasing through methods a bit.
For array-like, we would need to circumvent the automatic get_xy trigger.
The array option might actually be easier to implement but for me it feels better to set these up list-like. Obviously supporting spatial definition for inputs like this is tricky and we might need to look at allowing more granular specification of arguments at add_pars time to support mixed model input format types (i.e. specification of input specific get_xy function/spatial mapping rather than relying on what is on the parent object).