Save output files at user specified intervals

[h-vijayakumaran]

In cases of hyperelasticity where we need incremental load/displacement application, or in the case of rate or history dependent problems, the user would like to save output files at specific intervals. The current implementation 1cc776c makes this difficult as the solver handles the incremental loading internally and the solution saving step gets only the final solution. The user should be able to specify an output save (to disk) frequency and get the desired outputs.