Download data in dataexamples

[hrobarts]

Describe your changes

• Add REMOTEDATA class in dataexample.py for downloading remote data (for use in CIL demos and how-to examples How to demo notebooks for website CIL-Demos#144 )
• Move the internal data to an CILDATA class which has a default data directory
• REMOTEDATA, doesn't have a default data directory. get method must supply a data_dir pointing to the data. If the data isn't found, return an error suggesting to use the download_data method
• download_data method checks if the data already exisits in the data_dir, then requires user confirmation to start download

Describe any testing you have performed

Added tests in test_dataexample.py TestRemoteData class

• Mock the http response with a temporary zip file
• Mock the user input to accept the download

Link relevant issues

#1708

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License.
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[casperdcl]

lgtm; I'll probably open a follow-up PR slightly tidying the internal inheritance structure, but none of that is user-facing (wouldn't affect the docs)