Convert methods to `property` where appropiate

[ianspektor]

Methods with no arguments that just return a value from the class' internal state should be made properties via the @property decorator. This makes the syntax cleaner when calling the getter and makes it clear that the method is just returning one of the class' values.

We can additionally use the @property.setter where applicable.

Examples:

• core sampling: index, creator, set_creator
• core feature: name, dtype, sampling, creator, set_sampling, set_creator
  etc.

[diegomarvid]

Agree, schema() should be a property as well

[ianspektor]

schema() should be a property as well

not too sure about that, since it's creating and returning a new object