Add in the emissions filtering feature

[lkoolik]

When developing the emissions object, I added an input called filter_dict. The goal of the filter_dict is to allow the user to filter down emissions from a big emissions set (e.g., gridded EMFAC data by vehicle class and fuel type) and filter it down to specific categories. For example, if the user only wanted results from diesel trucks, they would use the filter_dict to reduce the fuel category to "diesel" and the vehicle category to "trucks".

Here are the steps I need to take:

• [x] Add a filter field to the control file.
• [x] Update the control file to read in the filtering dictionary.
• [x] Review the current filter_emissions function (part of the emissions object) to confirm that it works as expected
• [ ] Add this to the documentation