Accidental special characters from integers in regex

I'd expect any integer in the `<123>` format in a FiniteAutomaton regex to parse that integer. But certain integers conflict with already-defined [operators](http://www.brics.dk/automaton/doc/index.html) and therefore cause the FiniteAutomaton constructor to do something wrong:
- `FiniteAutomaton("<43>")` hangs (it gets converted to `"."`)
- `FiniteAutomaton("<59>")` hangs (it gets converted to `"@"`)
- `FiniteAutomaton("<117>")` throws an exception (it gets converted to `"~"`)

There are likely more integers that aren't hanging or throwing, but are getting converted to other operators (like `+`, `*`, `?`) and producing automata with the wrong behavior.

Have you considered putting a backslash in front of the character during the int parsing and conversion step in [`StringUtils.toCharExp()`](https://github.com/chocoteam/choco3/blob/master/choco-solver/src/main/java/org/chocosolver/util/tools/StringUtils.java#L84)? e.g. `"<43>"` would converted to `\.` and wouldn't trigger the behavior of an operator if it conflicts with one.