Change constructs access API for consistency

[davidhassell]

Current situation

The construct and construct_key methods take a single positional identity argument (https://github.com/NCAS-CMS/cfdm/blob/v1.8.8.0/cfdm/mixin/constructaccess.py#L301):

    def construct(self, identity, default=ValueError()):

Proposed change

The construct and construct_key methods take any number of positional arguments, as well as new keyword arguments:

    def construct(self, *identity, default=ValueError(), **filter_kwargs):

The **filter_kwargs are passed to the new Constructs.filter method for further selection flexibility.

This makes sense, as you can already match on multiple identities by providing a re.Pattern object, so it doesn't makes sense to restrict this to, say, *["latitude", "longitude"]

Consequences

The only possibly bad effect that this could have is for the default argument to be consumed by the *identity arguments if it is given as a positional parameter. For example, to Specify a default of None:

>>> f.construct('altitude', None)  # ok at v1.8.8.0
>>> f.construct('altitude', None)  # NOT ok with proposed change
>>> f.construct('altitude', default=None)  # ok with proposed change and at v1.8.8.0

Regardless of this potential for code breaking, I think this change will be beneficial.