Test that we convert actual measurements

... even if we can't do the interpolation, we should still use the
measurements either side.

https://www.pivotaltracker.com/story/show/76364260 [#76364260]
Fix convert_measurements

Always convert measurements even if we can't do an interpolation between
them.

https://www.pivotaltracker.com/story/show/76364260 [Delivers #76364260]
Merge pull request #51 from sealevelresearch/actual-measurements-always-converted

Fix convert_measurements