Strong types for arithmetic_range_validator

[eseiler]

Cuurent stable API:

    arithmetic_range_validator(option_value_type const min_) :
        min{min_},
        max{std::numeric_limits<option_value_t>::max()},
        valid_range_str{"[" + std::to_string(min_) + "," + std::to_string(max_) + "]"}
    {}

Todo:
strong types, e.g. greater_than, less_than, etc. in the constructor can help to increase readability. We need to see how they can be implemented:

• strong types sharg::greater_then (flies around in the main namespace..)
• strong types sharg::arithmetic_range_validator::greater_then (is very long...)
• designated initialisers sharg::arithmetic_range_validator{.greater_then = 3} (not sure if possible)