Merge branch 'master' of gitlab.psi.ch:OPAL/src into 318-optimizer-generalizing-objectives-with-sddsvariableat

bugfix: increase population by number of worker groups; arguments as members to avoid multiple getarg calls

revert usage of numeric conversion in parameter retrieval since these can be strings; fixes restart regression tests

better solution for #255: don't have to check whethre design variables occur in template file with every simulation run, just once

first steps towards #250; it remains to find a file format and dump the data to a file of this format

throw error if optimizer expression parser doesn't return successfully or if not whole expression has been parsed