Review the NVEnergyClient and fix if necessary

[r24mille]

As I cleaned up the unit tests for NVEnergyClient I noticed that the URLs constructed by the client do not exist at http://www.oasis.oati.com/NEVP/ Admittedly, I've investigated this very little but I don't think the client is functional:

1. It needs to be pointed at the new URL structure particularly for historical single day and monthly reports in the past.
2. It needs some integration tests written so it's easy to confirm that all URLs are actually responsive.
3. Update the affected unit tests accordingly.

[r24mille]

I looked into things a bit:

• Generation mix is not publicly available... or at least I couldn't find it on http://www.oasis.oati.com/NEVP/ The NVEnergyClient raises a NotImplementedError from BaseClient
• The get_load and get_trade integration tests fail with logs similar to:

  -------------------- >> begin captured logging << --------------------
  urllib3.connectionpool: DEBUG: Starting new HTTP connection (1): www.oasis.oati.com
  urllib3.connectionpool: DEBUG: http://www.oasis.oati.com:80 "GET /NEVP/NEVPdocs/inetloading/native_system_load_and_ties_for_12_20_2017_.html HTTP/1.1" 404 1245
  pyiso: ERROR: NEVP: request failure with code 404 for http://www.oasis.oati.com/NEVP/NEVPdocs/inetloading/native_system_load_and_ties_for_12_20_2017_.html, {}
  pyiso: WARNING: Unparseable data available in NVEnergy at 2017-12-20 for mode error: Empty DataFrame
  Columns: []
  Index: []
  urllib3.connectionpool: DEBUG: http://www.oasis.oati.com:80 "GET /NEVP/NEVPdocs/inetloading/native_system_load_and_ties_for_12_21_2017_.html HTTP/1.1" 404 1245
  pyiso: ERROR: NEVP: request failure with code 404 for http://www.oasis.oati.com/NEVP/NEVPdocs/inetloading/native_system_load_and_ties_for_12_21_2017_.html, {}
  pyiso: WARNING: Unparseable data available in NVEnergy at 2017-12-21 for mode error: Empty DataFrame
  Columns: []
  Index: []
  --------------------- >> end captured logging << ---------------------

So, the URLs being generated are wrong. All the load and trade data is available at: http://www.oasis.oati.com/NEVP/NEVPdocs/ElectricScada.htm