simplify the ClumSource's interface

@williamboxhall I find the current ```ClumpSource``` interface confusing. It requires the user to understand the differences between ```list```, ```get```,```getOrElse``` and ```apply```. Considering that the features introduced by the different methods are also provided by the ```Clump``` instances, I think we should remove them.

After the change, the source's interface is really simple, the user needs to use only ```get```.