Check momentum between `FROMFILE` distribution and BEAM

Summary

In the MR !513 (merged) we made that the BEAM command checks if energy or pc or gamma have been given, since the default P0 is not used anymore.

But this means that in the case where the distribution is FROMFILE, an energy needs to be given even though it should be read from the distribution file. For example this input file doesn't work anymore because there is no energy or gamma or pc in the BEAM command. It used to work because the default P0 kicked in, even though it was immediately replaced when teh distirbution file was read in.

Of course a work-around is to give any dummy energy, and it will be over-written when the distribution file is read in. But this will confuse users since they can give an energy different from the energy in the distribution file. The momentum specified in the BEAM command and in FROMFILE distribution type must be verified so that both values are consistent with each other.

Also, the regression tests that use FROMFILE still have PC in the BEAM command, that is why no regression tests had failed after this change.

TODO list:

• Add cross-check between BEAM command energy and FROMFILE energy (done in !562 (merged) )
• Update broken regression tests that used FROMFILE and the default P0 ( regression-tests#109 (closed))
• Update manual section 15.3.1 regarding FROMFILE type distribution. (OPAL/documentation/manual#76 (closed))