refactor(get_iv; get_all_ivs ): make parameter code handling more general for ivs

[ntdosch]

Addresses #8 -- Some updates to get_iv() and get_all_ivs() to handle parameter codes in a more general way and added sample_period and agg_method arguments to get_all_ivs(). Added helper function get_par_description() to extract the parameter description from sitefile_text. Parameter description is then used as the header of the data column in the returned DataFrame. Should also note that this PR doesn't touch the dv functions.

Added a few tests to test_nwis.py to make sure changes work properly (test_get_ivs_gw, test_get_ivs_sw, test_get_all_ivs_sw, and test_get_all_ivs_gw)

[codecov]

Codecov Report

Merging #10 (0774bc0) into develop (8d6dacb) will increase coverage by 0.82%.
The diff coverage is 82.50%.

@@             Coverage Diff             @@
##           develop      #10      +/-   ##
===========================================
+ Coverage    78.19%   79.01%   +0.82%     
===========================================
  Files           11       11              
  Lines          752      810      +58     
===========================================
+ Hits           588      640      +52     
- Misses         164      170       +6     

Impacted Files	Coverage Δ
pydrograph/nwis.py	85.12% <73.07%> (-0.89%)	⬇️
pydrograph/tests/test_nwis.py	100.00% <100.00%> (ø)

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