Add point by point to ManipulateReliabilityTable

[lucyleeow]

Second part of the work to add site & 'point_by_point' functionality to reliability calibration (as detailed elsewhere), similar to what's been done in EMOS: #1170. Follows on from #1658.
Work has been split into separate PRs to aid reviewing.

Acceptance criteria:

• ManipulateReliabilityTable is able to work point by point as well as on spatially aggregated data
• ManipulateReliabilityTable will work for both site and grid cubes
• Current tests pass
• New tests added for added functionality

cc @anja-bom @tjtg

[tjtg]

#1687 handles this, but didn't automatically close the issue here.
The PR description text used "addresses" when linking here rather than one of the magic github linking words.