Multi-step calculations with NWChem

[WardLT]

Is your feature request related to a problem? Please describe.
As with Molpro (see #198 ), you can perform multi-step workflows in NWChem to perform complex tasks more efficiently.
It would be excellent to be able to do common types of workflows (e.g., first do small then large basis, DFT then MP2) without much configuration on the user end.

Describe the solution you'd like
Add an option set to NWChemEngine that allows one to define a "pre" computation that gives a good initial guess for the next computation.

Describe alternatives you've considered
Setting the restart directories between two calculations to overlap, then trickery with the settings to have the second calculation look for files from the first.

Additional context