Implements a base metadata class that can be inherited from each service. This PR follows discussion from #109.

graph TD;
    nwis.NWIS_Metadata-->utils.BaseMetadata;
    wqp.WQP_Metadata-->utils.BaseMetadata;

Changes in dataretrieval.utils:

• BaseMetadata class is implemented such that it is initialized from the a request (md = BaseMetadata(request)). However, each service should inherit from BaseMetadata and implement their particular methods.
• The set_metadata function is removed, this is handled by the BaseMetadata's methods.

Changes in dataretrieval.nwis:

• NWIS_Metadata class inherits from utils.BaseMetadata. It implements site_info and variable_info as properties and handles the metadata comments.
• User sees a nicer __repr__ of the Metadata object.

Example:

In [1]: from dataretrieval import nwis

In [2]: df, md = nwis.get_ratings(site="01594440")

In [3]: df
Out[3]: 
    INDEP       DEP STOR
0    2.99     30.00    *
1    4.00    110.00    *
...
9   20.85  16497.75    *
10  27.90  31100.00    *

In [4]: md
Out[4]: NWIS_Metadata(url=https://nwis.waterdata.usgs.gov/nwisweb/get_ratings/?site_no=01594440&file_type=base)

Changes in dataretrieval.wqp:

• "WQP_Metadata" inhertis from utils.BaseMetadata and implements site_info property.

Example:

In [1]: from dataretrieval import wqp

In [2]: df, md = wqp.get_results(lat='44.2', long='-88.9', within='0.5')

In [3]: df
Out[3]: 
   OrganizationIdentifier                     OrganizationFormalName  ActivityIdentifier  ... DetectionQuantitationLimitMeasure/MeasureUnitCode PreparationStartDate  ProviderName
0               WIDNR_WQX  Wisconsin Department of Natural Resources  WIDNR_WQX-35940585  ...                                               NaN                  NaN        STORET
1               WIDNR_WQX  Wisconsin Department of Natural Resources  WIDNR_WQX-35940585  ...                                               NaN                  NaN        STORET
...
17              WIDNR_WQX  Wisconsin Department of Natural Resources  WIDNR_WQX-35940585  ...                                               NaN                  NaN        STORET

[18 rows x 63 columns]

In [4]: md
Out[4]: WQP_Metadata(url=https://www.waterqualitydata.us/data/Result/Search?lat=44.2&long=-88.9&within=0.5&zip=no&mimeType=csv)

Changes in tests:

• Added tests for the BaseMetadata class
• Modified tests where md.site_info and md.variable_info were expected as callables

Breaking changes:

• md.site_info and md.variable_info are no longer callables but class properties. I am not sure why those were implemented to return lambda functions, but my guess is so the service query was only made if they where explicitly called by the user. If that was the case, the @property decorator should take care of that.