new weldx validators

[CagtayFabry]

now that the API is mostly stable and we have found a good way to attach custom validation behavior to WeldxConverter it should be easy to implement more custom validators.
maybe something like wx_min & wx_max
or even a wx_range that combines min/max checks for numeric values (like wx_range: [0, 3.14])

Do you have any further ideas?