Insight units followup

This PR sanitizes the Pojos; it replaces the rather purely named methods like getEmissionWavelengthAsLength() (introduced by my previous PR #3190 ) with getEmissionWavelength(UnitsLength unit), where the client can request a specific unit (or get the "original"  unit as it comes from the DB, if passing 'null' as unit argument). 
This also makes handling the Pojos compliant to the Python gateway currently implemented by @will-moore .

Test: Basically the same as for PR #3190 : Make sure the metadata is displayed like before the units work; With @joshmoore 's PR #3227 now also the 'alternate' image test files can be used for reviewing this PR.

/cc @pwalczysko @jburel