Add support to load DMs from a file path

File paths can now be loaded using the `from_file` method, the documentation was changed to reflect this and I added some test data to the module.

I added a note about file handles remaining open if they are passed in to the method.

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I ended up copying a function from maths/stats/ordination/tests/test_ordination.py to retrieve a file path given a filename. Not sure if we should factor this out and put it in utils or leave as is.

The `setup.py` file will need to be changed once #251 is merged.