parse_cf should not overwrite existing axis attributes

[jthielen]

This was brought up in #960: while the current re-assignment of the axis attribute in parse_cf() keeps parse_cf() idempotent, it may still overwrite information that was previously in that attribute. Since the issue of #960 itself was resolved by #967, repeated below are the parts relevant for this issue so they don't get lost. Any thoughts about the best way to fix this?

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Current implementation:

MetPy/metpy/xarray.py

Lines 336 to 343 in 1a052be

	def _assign_axes(coord_map, var):
	"""Assign axis attribute to coordinates in var according to coord_map."""
	for coord_var in var.coords.values():
	if 'axis' in coord_var.attrs:
	del coord_var.attrs['axis']
	for axis in coord_map:
	if coord_map[axis] is not None:
	coord_map[axis].attrs['axis'] = axis

Some ideas for possible fixes:

• Use a metpy_axis attribute instead of axis (or some other "namespaced" attribute)
• Store the axis type -> coordinate information in the attributes of the crs coordinate
• Store the axis type -> coordinate information in the CFProjection object itself