parcel_model: Moving-grid Parcel Model for Aerosol Activation

This milestone denotes the final development of the original moving-grid parcel model intended for aerosol activation studies. It has a module of activation parameterizations which can be compared against the parcel model, and has all the features necessary to do idealized activation experiments. Furthermore, it is nearly identical to the version used to generate the activation scheme of Rothenberg and Wang (2015, in prep) (the only changes relate to IO facilities and some superficial changes to make the model simpler to run).

Further refinements and bugfixes to this scheme will issued on the branch moving_grid. However, future development on the parcel will deviate sharply form this formulation in order to introduce more sophisticated microphysics.