Introduce range syntax

[Raduh]

MWS can now process ranges in the following form:

<mws:range low="<double_low>" high="<double_high>"/>

We should be able to convert from latex-style syntax for ranges (see below) to the format above.
An intuitive latex-style syntax for ranges could be:
?[<double_low> - <double_high>]

[kohlhase]

I think that range variable also should have a names (and return substitutions) I propose

<mws:range name="foo" low="<double_low>" high="<double_high>"/>

and

?{foo}[<double_low> - <double_high>]