Add Identifyflats

[Teschl]

Implements identifyflats for the GridObject. To be able to use this function efficiently when creating the FlowObject I added an argument raw to just return the ndarray.

Since there is no way of making a function private in python, I think it's important to still add a function that returns multiple GridObjects. By using the output argument, the number of returned dems can be changed.

• flats, sills = dem.identifyflats(output=['flats', 'sills']) (default)
• flats = dem.identifyflats(output=['flats'])
• sills = dem.identifyflats(output=['sills'])

When using dem.identifyflats(raw=True) instead of multiple GridObject the return will be an int32_t nd.ndarray. This also contains presills.

Closes #22

[Teschl]

Adjusted the documentation and resolved the merge conflict, should be ready to be merged now. @wkearn