Make run() method return table/grid-tuple instead of dict

Hi guys, working on the create grid method, I realized that the run() method could just return both the table and the grid. This allows us to get rid of the constants for the dict keys and also simplifies the call code

table, grid = method.run(...)

vs.

result = method.run(...)
table = result[BeirleEmissionsCalculator.TOTAL_KEY]
grid = result[BeirleEmissionsCalculator.GRID_KEY]