Expansion from restart file

Since !538 (merged), it is possible to use an adaptive polynomial order procedure as the simulation progresses. However, restarting these cases is not trivial, since at the moment we need to prescribe the polynomial order manually (arranging the elements in composites and setting the order on each composite).

It would be nice to have the possibility of loading the expansion information directly from a field file. This could look like:

 <EXPANSIONS>
    <F FILE="file.rst" />
 </EXPANSIONS>