Adds a results aggregator file

[samgdotson]

This PR adds a script to aggregate ReEDS results.

In order to combine datasets from all scenarios:

1. All of the scenarios need to be in the same parent directory
2. clone this repository using git clone https://www.github.com/ucsusa/reeds-analysis.git (git must be installed)
3. The directory structure should be the following

parent_directory/
    |- scenario 1
         |- outputs
    |- scenario 2
         |- outputs
    |- scenario 3
         |- outputs

4. From the command line run

python path/to/reeds-analysis/scripts/results_aggregator.py --directory path/to/directory/with/scenario_results

The result will be new folder called combined_results. Each file therein will be unique with a column for the scenario the data came from.