added xclim subset module

[cehbrecht]

Changes:

• This PR adds the subset module with tests from xclim.
• The tests are using test-data from the submodule: https://github.com/roocs/xclim-testdata.
• Installation doc updated with notes for conda environment and submodule initialization.

[cehbrecht]

@ellesmith88 I have moved test_subset.py to test_operations.

[cehbrecht]

@ellesmith88 I have tested like this:

$ git submodule update --init 
$ conda env create -f environment.yml
$ conda activate clisops
$ python setup.py develop
$ pip install -r requirements_dev.txt
$ pytest tests/

All tests passed ...

I will try to fix travis with a new PR (see #2)

[ellesmith88]

@cehbrecht Thanks, I realised I was doing something wrong with the testing. I'm happy with it.

[cehbrecht]

@ellesmith88 merged. Thanks for review :)