Refactor package

[Teschl]

I refactored the package, so the locations of different functions make more sense. The functions creating a GridObject are now in utils.py, and calling topotoolbox.GridObject() will result in an empty GridObject.

I also changed the way autosummary is used, so the way the functions are listed reflects the way they are used. Instead of having topotoolbox.utils.load_dem() in the documentation, it's now topotoolbox.load_dem() for example. The docstrings are now in the numpy format instead of the Google one, this keeps readability while insuring they are formatted right in the documentation.

closes #34

[wkearn]

Good work, @Teschl. I left one comment about some outdated documentation.

When I went to build the docs, I got some errors because my old _autosummary directory still had files from the older version. I guess make clean just cleans the _build directory. Do you know of a way to make sure that those _autosummary files also get deleted automatically? It's not a big deal, because I can always manually delete that directory. It's just a little annoying to have to remember to do that.

[Teschl]

@wkearn I added 'clean' to the makefile, overwriting the Sphinx version. It now cleans _build, _examples and _autosummary. After building, the _autosummary directory will get cleaned as well.