Refactor read modules

[OnnoEbbens]

As discussed here: #47

refactor all get_.. functions in the read modules to:

• download_..
• optionally:
  • discretize_..
  • get_..

[OnnoEbbens]

@dbrakenhoff: this is a first attempt and not finished. I mainly created the PR to have a first idea on how the refactoring turns out. I won't work in this the coming weeks so feel free to add to this PR if you want.

[OnnoEbbens]

The aim of this PR is to make a more clear distinction in functions for downloading and discretization.

1. Functions that download data start with: download_..
2. Functions to discretize data spatially and temporally start with: discretize_..
3. Functions that do both, start with: get_..

I implemented this for a few read modules already, a good example is the ahn module.

A lot of functions in the read modules will be renamed in this PR. @rubencalje, @bdestombe, @dbrakenhoff could you check if you agree with the new setup? Then I will finish it for all read modules soon.

More info in this issue: #47

[dbrakenhoff]

Some minor (nitpicky) comments here and there, but otherwise looks really nice. I think this is much clearer, and helps separating (down)loading data from converting it to model input.

Some function names get a bit longer unfortunately, but that's the price of clarity I think, and we have auto-complete for that :).