Make mexa more friendly in ExaHyPE

Currently the applications which use the meta-specfile parameter system (i.e. CCZ4 and GRMHD) have small childhood diseases:

• They are very verbose in output at startup. mexafile::toString() is kind of too noisy and should instead print something more human-readable (such as foo = bar [ref] instead eq(foo, bar, ref)), probably one tarch:::logInfo per line.
• In the current usage, foo = mf[key].as_double() where mf is a mexafile, if the key is in the wrong data type (in this example: not castable as double), it spills out an error message without any reference where the error occured, only way to find it is to start with gdb and to go up the stack trace. This is exactly against the philosophy of mexa where the parameter source should always be dragged along, also with the mexa::value type. This needs to be implemented and tested.