Wms default time

[SpacemanPaul]

Addresses #673

Can now add to layer config:

   "default_time": "latest",    # Default behaviour - use the most recent available date as the default time, OR:
   "default_time": "earliest", # Use the earliest available date as the default time, OR:
   "default_time" : "2020-01-01",   # Use an explicitly defined date as the default time.
                                                # If the explicit date is NOT a valid date for the layer, a warning is raised, and 
                                                # the layer reverts to the default behaviour ("latest").
                                                # Note that this warning will be raised repeatedly on dynamic layers.

The default date is only published in metadata by WMS.

The default also applies to WMTS, and WCS1 requests with no user-selected time, but there is no way for a client to know what the default date is.

WCS2 requests with no dates select ALL available dates, as per the spec.

NB. Also fixes breakages introduced by xarray 0.19.0

[codecov]

Codecov Report

Merging #676 (1c1a7b4) into master (8687c50) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #676      +/-   ##
==========================================
+ Coverage   90.73%   90.76%   +0.02%     
==========================================
  Files          43       43              
  Lines        5691     5708      +17     
==========================================
+ Hits         5164     5181      +17     
  Misses        527      527              

Impacted Files	Coverage Δ
datacube_ows/ows_configuration.py	96.79% <100.00%> (+0.05%)	⬆️
datacube_ows/styles/hybrid.py	90.90% <100.00%> (ø)
datacube_ows/styles/ramp.py	74.21% <100.00%> (ø)
datacube_ows/wcs1_utils.py	96.19% <100.00%> (ø)