Remove temporaries from Variational Propagators

Solves the following issue(s):

Closes #265 (closed)

Core changes:

None

Model-specific changes:

Optimize memory allocation for the variational propagators by creating some arrays at the beginning of the simulation that can be use after to avoid allocating temporary array. This could probably help to avoid out-of-memory errors in when running the code in raven.

EDIT : I was indeed able with this branch to run on 4 procs on raven. I ran the "Orszag-Tang vortex" test case (same as in my paper) and the results are similar, but since I am now using Newton algorithm I could use time step larger (I used 20 time larger then when using the Picard iteration as in the paper) which allow for a much faster solve (the solve took only 20 minutes when it last several hours in my previous code!)

Documentation changes:

None