Flag possible incorrectly defined time coordinate variable

[RosalynHatcher]

Time coordinate variables must have one or more of the following:

• axis attribute has the value 'T'
• Units of reference time
• The standard_name attribute is one of 'time' or 'forecast_reference_time'

If a coordinate variable with time units but none of the above is contained in a file which does not have any other time coordinate variable present it is likely this variable may have been mis-defined. Issue an information message in this case.

See test file CRM033_test1.nc for an example.

[RosalynHatcher]

See also #49