call spots + ref spots variables now saved as zarrays to save memory …

[paulshuker]

…and nb load time. Completes issues #94 and #95.

Note: This will remove backwards compatibility with older v1 test versions from call spots onwards.

Checklist

• [x] I have updated the changelog.txt, if applicable.
• [ ] I have bumped the software version in _version.py, if applicable.
• [x] I have checked that the unit tests passed.
• [x] I have checked that Minnie and/or RoboMinnie passed.
• [x] I have resolved merge conflicts.