Update expcetion message in excBadVarName to make it more clear what the problem is
I got confused by this errro:
terminate called after throwing an instance of 'SG::ExcBadVarName'
what(): SG::ExcBadVarName: Bad name for auxiliary variable `2022_particle'. Variable and class name must be identifiers, and variable name must not be empty.
Then I realised the problem was the first character cannot be a number, but that was not clear from the message.