Consider hosting test data externally

[jat255]

The repo size for ETSpy is ~63MB when compressed, which is quite large. The vast majority (99.6%) of this is the test datasets, a couple of which are also used for get_needle_data() and get_catalyst_data().

This isn't too hard to set up (see KikuchiPy as an example: https://github.com/search?q=repo%3Apyxem%2Fkikuchipy%20pooch&type=code , https://github.com/pyxem/kikuchipy/blob/a016c0a38b84cc74b4110c517807504e5f248c50/src/kikuchipy/data/_data.py#L22, and https://github.com/pyxem/kikuchipy/blob/a016c0a38b84cc74b4110c517807504e5f248c50/src/kikuchipy/data/_registry.py)