Add logging to improve upon if verbose: print(<msg>) statments

[sadielbartholomew]

Use the Python built-in logging module to manage messaging to the user, in particular to add finer configurable granularity of detail and/or severity of said messages, and consolidate the code so that it doesn't have to wrap anything to print with if verbose statements, like so:

cf-plot/cfplot/cfplot.py

Lines 1122 to 1128 in a325942

	if verbose:
	if ptype == 2:
	print('con - making a latitude-pressure plot')
	if ptype == 3:
	print('con - making a longitude-pressure plot')
	if ptype == 7:
	print('con - making a time-pressure plot')

As with cf-python, I think it would be useful also here to add another level as a custom one to extend the five provided, which indicates extra detail as opposed to just increased severity (see https://github.com/NCAS-CMS/cf-python/blob/de6d0b5cf9d52aa0d0302d76e68fdfab94a62004/cf/__init__.py#L359-L370).