allowed values when working with integers

[stijnvanhoey]

Allowed values compares strings, but in the yaml-specification users can actually provide integer values as well, e.g.

uncertainty: 
    allowed: 3650

The yaml-parser will interpret this as a dictionary with an integer value leading to issues on the validation, whereas

uncertainty: 
    allowed: '3650'

will compare the string values as such.

Either we interpret all allowed specifications as a string (nevertheless the type) or we could argue to the user that for numbers, the following specs are more appropriate:

uncertainty: 
    numberformat: x
    min: 3650
    max: 3650

@peterdesmet ?