Allow for maximum one standard method of each type-DTO combination

In the current Service DSL multiple instances of the same standard method (e.g. crud methods) can be defined. For example on ecan define two update methods for one CustomerDto. This amount of freedom is not useful, because the code for these standard methods is completely generated, and this allows for multiple generation of identical code. Therefore we will remove the possibility to have more that one instance of a specific standard methods.

The advantage is that we can then automatically link from the Presentation model to the standard methods (there is only one, you don't need to make a choice) which helps to make the Presentation DSL easier to use as well.