DOC: update Changelog ready for release

First pass at Changelog - anyone remember stuff I missed?

Any more credit that should go in there?  Please add yourself if I
forgot you.
RF: check separately for Nones to avoid warning

Avoid future warning by checking for two Nones with separate if clauses.
Merge pull request #284 from matthew-brett/none-none-fix

RF: check separately for Nones to avoid warning

Avoid future warning by checking for two Nones with separate if clauses.
RF: docstring rewrite; a PEP8 fix

Suggested by Eric L from PR review.
Merge pull request #276 from matthew-brett/in-memory

MRG: API to help modify image caching

Add 'caching='fill'|'unchanged' keyword to get_data method.

Add in_memory property to tell user whether image already has an in-memory copy of the data.
Merge branch 'pre-release-changelog' into main-master

* pre-release-changelog:
  DOC: update Changelog ready for release
DOC: update changelog

With new API for caching, parrec2nii changes.
DOC: `get_data` docstring rewrite

Rewrite of docstring for get_data to give more details on the cache.