Improve results API

[fcooper8472]

A few convenience methods to add to the results api:

• [x] Given a sector model name, return a list of output names
• [x] Given a model run name, return a list of sector models
• [x] Given a model run name return a list of names of scenarios
• [x] Given a scenario name, return a list of names of scenario outputs

Also:

• [x] Add the scenario information to results.available_results()

[fcooper8472]

Question for @willu47:

Scenarios are in the model run like so:

scenarios:
  population: population_med
  climate: central
  reservoir_level: historical
  water_sector_energy_demand: central

You want a list of scenario names given a model run... do you want:

['population', 'climate', 'reservoir_level', 'water_sector_energy_demand']

or

['population_med', 'central', 'historical', 'central']

or something else?

[fcooper8472]

Perhaps just return the dictionary?

[fcooper8472]

First draft is ready - @willu47 I'll open a PR for your comments on the interface

[willu47]

Closed by #374