Feature/is simple

1. fix problem with non-running tests commented by Barend
2. make is_valid testing more elaborate (for each open areal geometry test the corresponding closed,
    open with reversed points order and closed with reversed point order)
3. move implementation for number of distinct values in a range in separate file
4. re-design is_valid for linestrings, which also fixes a bug (the bug was producable by the test cases
    recently added in the is_valid unit test)