Compiler doesn't support numerical Maps with negative output values

[langosco]

Currently, the compiler doesn't support programs with numerical Maps that have negative output values for some inputs (cf PR #13).

For example, the program

 out = rasp.numerical(rasp.Map(lambda x: x - 1, rasp.tokens))

raises an error when compiled with vocab={0,1,2} (or any vocab including values <= 0).

It would be nice to support this; for example, negative values could be shifted by a bias parameter.