CFstore subspacing

[georgeoobrienNCAS]

Implement subspacing for axes queries (i.e. subset of months over time)
Important ones are time, latitude and longitude so just do those.

[davidhassell]

I've just been checking the time subspacing syntax, and thought I'd share for the record. cf.cellwi is probably the query of choice here (and for the X and Y axes, too).

>>> import cf
>>> f = cf.example_field(2)
>>> print(f)
Field: air_potential_temperature (ncvar%air_potential_temperature)
------------------------------------------------------------------
Data            : air_potential_temperature(time(36), latitude(5), longitude(8)) K
Cell methods    : area: mean
Dimension coords: time(36) = [1959-12-16 12:00:00, ..., 1962-11-16 00:00:00]
                : latitude(5) = [-75.0, ..., 75.0] degrees_north
                : longitude(8) = [22.5, ..., 337.5] degrees_east
                : air_pressure(1) = [850.0] hPa
>>> print(f.subspace(T=cf.cellwi(cf.dt(1959, 12, 1), cf.dt(1960, 5, 1))))
Field: air_potential_temperature (ncvar%air_potential_temperature)
------------------------------------------------------------------
Data            : air_potential_temperature(time(5), latitude(5), longitude(8)) K
Cell methods    : area: mean
Dimension coords: time(5) = [1959-12-16 12:00:00, ..., 1960-04-16 00:00:00]
                : latitude(5) = [-75.0, ..., 75.0] degrees_north
                : longitude(8) = [22.5, ..., 337.5] degrees_east
                : air_pressure(1) = [850.0] hPa

In this example cf.wi would give the same result but that is not always the case, since cf.wi queries representative cell values, rather than bounds extents.