Minor HDF5 refactor and drop the rotational DOFs

[kmokstad]

Adding two convenience methods doing some repetitive tasks, also using std::to_string() instead of std::stringstream (more compact). Then removing the three rotational DOFs if there are 6 DOFs per node (assuming the 3 rotations are ordered after the translations per node. They are not needed for visualizing deformations.

May consider adding them purely as scalar fields instead?

Not tested (since the current version of SISO only supports hex elements).