A few minor issues with get_iv() and get_all_ivs()

[ntdosch]

I was using the get_all_ivs() and noticed a couple of minor items that could be cleaned up or improved:

1. The data aggregation arguments available in get_iv() are not available in get_all_ivs(). Could modify get_all_ivs() to accept all arguments accepted by get_iv() (specifically sample_period, and agg_method).

2. Discharge parameter and units are hard coded in get_ivs(), and are included in the returned DataFrame even when a user pulls gw level data. One fix would be to add a parameter code/info dictionary to nwis.self to use in the method so that the appropriate units are added to the data column to the returned DataFrame.

3. Might consider adding a few additional parameter types to self.parameter_codes, specifically: 62611 (Elevation, GW, NAVD88) and 00065 (Gage height, feet).