Separation between python solvers and external solvers (FEnics, ABACUS etc..)

[atulag0711]

Hi,
In the current integration tests, we have python based algebraic forward "solvers". I think this can be extended and also the forward model base class needs to be extended. Some points:

1. More classical algebraic python based tests can be added (https://www.sfu.ca/~ssurjano/uq.html).
2. There should be a provision to have a custom forward base class for external solvers (FEM, FD or anything, ultimately just an input and output map).

Thoughts?