Some fixes to the tomask and private _tomask methods to work properly when an empty list of location is received.  It will now return an Np/Nt array of False.  Also made some tuneups to network/models/pore_topology which revealed the bug in tomask in the first place.
Some changes to the linear solver structure.

Nearly all of the changes have been made in the behind the scene. The
"setup" and "solve" method has been added which might help in the case
someone wants to use another solver.
Also test_script is debugged.
Update documentation for the transport algorithms.
Minor changes to the "import" in generic methods.
Merge branch 'develop' of https://github.com/PMEAL/OpenPNM into develop
Update to add_models to allow for models that are ONLY generated upon specific request.  This also required a change to the regenerate method so that it looks to see if a model has been marked for 'on_demand' regeneration and removes that from the list of model it regenerates.
Fixed bug/typo in regenerate...not sure how I missed it.
Merge branch 'develop' of https://github.com/PMEAL/OpenPNM into develop