Replace allVariables with a multimap

[janez]

For variables to match, they must have the same varType and name. Since all variables are, in principle, constructed through TVariable::make, lookups take O(n^{2) when, say, loading a data file. This may be quite significant with 10000 variables.}

We can replace allVariables, which is currently a list, with a multimap indexed by varType and name.

Alternatively (and perhaps better), we can have separate multimaps for each varType; multimaps key would then be a name.

[janez]

Separate multimaps for different value types