Add support for features taking only numeric/integral values.

[jurko.gospodnetic@…]

It should be possible to define a feature with a limited but large set of values such as 'all positive integral values' or 'all integral values'. Currently it is only possible to declare a free feature with no value checking or features with a predefined set of values.

A possible implementation would be to allow defining features with a validation rule that would be used to validate the entered value.

As a quick-fix users can workaround this issue in the following ways:

• By defining a feature with a huge range of predefined values but that seems like a waste of resources.
• By defining only a subset of all the possible values and expect the used to extend the feature value range using the feature.extend rule when he needs to but this unnecessarily complicates user code.
• By defining a feature as free and then adding a 'run-time' check in the location (rule or action) where the feature value is actually used. This works but potentially duplicates code and detects invalid feature values later than necessary.