day of year constraint
review actions
String serialization of collapsed coordinate.
ENH: Fields file loader grid type extended interp.

The fields file loader now handles grid type 29 and 27 correctly and
also interprets specifically the horizontal grid type variable when set
to global.
MAINT: Review actions made.
Merge pull request #849 from cpelley/vector_coord_lbc

ENH: Fields file loader grid type interpretation extended.
ENH: Scalar coord promotion util.
Merge pull request #848 from cpelley/scalar_time_netcdf

ENH: Scalar coord promotion.
Merge pull request #847 from bjlittle/collapse

String serialization of collapsed coordinate.
Merge pull request #839 from bblay/dayofyear

Added experimental a "day of year" constraint.
Work in progress implementation of better datetime constraints.
Added tests for PartialDateTime.