Remove temporary in variational propagators
Bug description / feature request:
The Varational propagators are creating a lot of temporary arrays for the evaluation of the different quantities in the Newton method.
This creates an out of Memory error when running VariationalMHD
on Raven (or any other cluster I guess).
Expected behavior:
Being able to stay in memory bounds by not allocating arrays at each iteration
Proposed solution:
Create and use enough array at the beginning of the run and use them after to reduce the memory footprint.