Add `.cf.formula_terms`

[dcherian]

With ROMS, here's what the parametric coordinates look like

        double s_rho(s_rho) ;
                s_rho:long_name = "S-coordinate at RHO-points" ;
                s_rho:valid_min = -1. ;
                s_rho:valid_max = 0. ;
                s_rho:positive = "up" ;
                s_rho:standard_name = "ocean_s_coordinate_g2" ;
                s_rho:formula_terms = "s: s_rho C: Cs_r eta: zeta depth: h depth_c: hc" ;
                s_rho:field = "s_rho, scalar" ;
        double s_w(s_w) ;
                s_w:long_name = "S-coordinate at W-points" ;
                s_w:valid_min = -1. ;
                s_w:valid_max = 0. ;
                s_w:positive = "up" ;
                s_w:standard_name = "ocean_s_coordinate_g2" ;
                s_w:formula_terms = "s: s_w C: Cs_w eta: zeta depth: h depth_c: hc" ;
                s_w:field = "s_w, scalar" ;

I think the only way this makes sense is if we have

ds.cf.formula_terms = {
    "s_rho": {"s": ds["s_rho"], etc},
    "s_w": {"s": ds["s_w"], etc.}
}

Note we cannot use standard_name as keys because the standard name is the same for both s_w and s_rho.

For DataArrays we can also have

ds.s_rho.cf.formula_terms = {
    "s": ds["s_rho"], etc...
}

We just need refactor the terms dictionary from here:

cf-xarray/cf_xarray/accessor.py

Lines 1687 to 1704 in fd7e486

	if (
	"formula_terms" not in ds[dim].attrs
	or "standard_name" not in ds[dim].attrs
	):
	continue
	formula_terms = ds[dim].attrs["formula_terms"]
	stdname = ds[dim].attrs["standard_name"]
	# map "standard" formula term names to actual variable names
	terms = {}
	for mapping in re.sub(": ", ":", formula_terms).split(" "):
	key, value = mapping.split(":")
	if value not in ds:
	raise KeyError(
	f"Variable {value!r} is required to decode coordinate for {dim} but it is absent in the Dataset."
	)
	terms[key] = ds[value]

Note the apparently more permissive implementation for a DataArray here:
https://github.com/NOAA-GFDL/MDTF-diagnostics/blob/70622b2fb9395de3a8ac3bc92c55c955e3d489b2/src/xr_parser.py#L264-L275

xref NOAA-GFDL/MDTF-diagnostics#186