Handle "nan", "inf" or "-inf" in formula expressions

At the moment, to set a variable equal to nan, or inf in the config file's variable definition modules is only possible through things like sqrt(-1) or -log(0) etc, both of which are exceptionally hacky and ugly, and produce a warning message when run.

This will require some connection to numexpr which is the main "backend" to the formula parsing. This issue on github is very related: https://github.com/pydata/numexpr/issues/298, and it might be connected to issue #18 as well.