Add "delta" table processor

Returns the change compared with the previous period.

Only for non-date-indexed collections for now i.e. where the x-axis key is not a timestamp.

Simplified the code to test if a key is Date-y to match logic used when parsing data, and re-used function for both.
Add delta columns to help usage tables
Expanded and renamed test for parsing collections

The name of the test was terrible, but covered the basic cases for non-date-indexed datasets.

Expand out the tests so that they cover the new cases where datasets with a duration of > 1 copy their value to the top level, but leave the values array in place.
Defend against undefined axes options in collections

Some modules do not need to have any axes defined e.g. KPI modules, and so we can't assume that these will exist when parsing backdrop responses.
Merge pull request #717 from alphagov/feature/table-deltas

Feature/table deltas