Add simplifiedExpression()

This simplifies the expression if possible:

- Mo 08:00-13:00; Tu 08:00-13:00
+ Mo,Tu 08:00-13:00

Doing this inside normalizedExpression() would create many more Osmose errors than necessary. Better differenciate normalization (which helps detecting invalid input) and simplification (which is more about setting the best possible expression if we're modifying it anyway).

This comes from the fact that I got comments on changesets where I was setting a non-simplified expression. It's all the same to computers, but humans find the simplified version more readable/logical.