Updating `__str__` method of `GammaDistribution`.

Returning Exponential distribution string if the shape attribute
is 1.
Updating `DMCSimulationResults` class.

Adding `write_result_summaries` method to walk through all result
directories, parse results, and write them to a tab-delimited text file
in each respective result directory. Also adding additional attributes
and methods to support the `write_result_summaries` method.

Adding test of new method to `test_parsing.py`.
More work on `PsiPowerPlotGrid` class.