PostCCO2 specs "diet", Part 1

Compare the specs for `PostCCO2.to_entity` with those for
`PostCCO2.from_entity`. Which are clearer, easier to read, more
navigable? Part 2 will rework the `to_entity` specs in line with the
style of those for `from_entity`.

RSpec: 820 examples, 0 failures (but see Issue #87)
RuboCop: 153 files inspected, no offenses detected