Add math syntax to parameter files

The toml specification does not allow any other varialbe types then Integers, Floats, Strings, Vectors of those and maybe dates, cf. https://toml.io/en/v1.0.0

Sometimes it might be convenient to do some simple within within the parameter file, although, that should be kept to a minimum (or should it?) To this end, we have to work around the toml specs. One way is to use the syntax "$( )" to instruct the parser to interpret stuff between the braces as julia code. Of course, we should not allow arbitrary code execution with that. On the other hand, whoever is trying to do that could also just fire up a normal julia session and run stuff there.

Example syntax

some.variable = "$(x = 1; y = 2; x + y)"

should be equivalent to

some.variable = 3

An advanced feature to add would be to allow accessing other variables from the same parameter file in such an expression. The only with this is that we have to settle on a syntax for how to access those variables, because I think the expanded toml keys a la option1.set2.variable3 does not play nicely with other arithmetic sugar (e.g. broadcasting). Or would it be feasible to scan all expressions for any of such options and then just replace them with a temporary name for the evaluation?