BeamCXPEC interface class is missing the donor_metastable attribute used in BeamCXLine

[vsnever]

The BeamCXLine model uses the donor_metastable attribute of the BeamCXPEC.

core/cherab/core/model/beam/charge_exchange.pyx

Lines 309 to 312 in 9ae5a3e

	self._excited_beam_data = []
	for rate in rates:
	if rate.donor_metastable == 1:

However, the interface class of the BeamCXPEC does not have this attribute.

core/cherab/core/atomic/rates.pxd

Lines 53 to 55 in 9ae5a3e

	cdef class BeamCXPEC:
	cpdef double evaluate(self, double energy, double temperature, double density, double z_effective, double b_field) except? -1e999

The attribute is added only in the openadas implementation of the BeamCXPEC.

core/cherab/openadas/rates/cx.pxd

Lines 23 to 29 in 9ae5a3e

	cdef class BeamCXPEC(CoreBeamCXPEC):
	cdef readonly:
	dict raw_data
	double wavelength
	int donor_metastable
	Function1D _eb, _ti, _ni, _zeff, _b

First, models should not use any attributes/methods that are not in the interface classes of atomic rates.

Second, because NullBeamCXPEC does not have a donor_metastable attribute, running code with missing CX PECs for some species will raise an AttributeError if atomic data is configured with missing_rates_return_null = True.

There are two possible solutions. The first is to add the donor_metastable attribute to the BeamCXPEC interface class, and the second is to update the AtomicData.beam_cx_pec() method to return a list of (donor_metastable, rate) pairs instead of a list of rates. I think the first solution is less invasive and will have less impact on user code.

@jacklovell, @Mateasek, which solution do you like better?