Configure attribute names that provide time coverage start and end

[forman]

Users are currently forced to write a custom pre-processor if time coverage start and end is not given by CF attributes time_coverage_start and time_coverage_end. This can be avoided by simple configuration.

Proposal

input:
  # Names of global attributes that provide time coverage start and end.
  # Yields `time_bnds` coordinate pair and `time` timestamp (the pair's center value)
  time_bnds_attr_names = ['time_coverage_start', 'time_coverage_end']
  # The name of the global attribute that provides the data's timestamp.
  # If not given the `time_bnds` pair's center value is used.
  time_attr_name = null

Example:

input:
  time_attr_name = 'observation_date'
  time_bnds_attr_names = ['start_date', 'stop_date']