Add "long_name" field to measurement_type. gh-6 gh-7

- New "long_name" field is added to the measurement_type model, and it is
  added to the filtering and ordering options in the api resource.

- The map interface shows users a list of these long names rather than the IDs

- In the ingestion file, a new subroutine units_and_name is created with
  units and names defined in one place, so it is easier to add new measurements
  in future. Additionally, it uses a dictionary rather than a long list of elseif
  statements to be a bit more readable.

- Measurement type IDs (eg Rho_wn_IS) are now stored as all lower case, due to
  some inconsistency in the input files which could lead to duplicate entries
  with only case differences.