added enumeration to _atom_type_scat.length_neutron

[rowlesmr]

requires COMCIFS/Enumeration_Templates#16
see also COMCIFS/Enumeration_Templates#1

Simple lookup and maintenance, dealing only with element symbol and mass number, not the myriad _atom_type.symbol values.

[vaitkus]

Allowing complex values seems like the correct way to go. However, given that we currently interpret Complex numbers as necessarily containing the imaginary component (e.g. 5 + 0i), the type change from Real to Complex could potentially invalidate some of the older entries (the data name was defined in CIF 1.1). This might actually be an argument in favour of relaxing requirements for Complex numbers.

In the COD, the _atom_type_scat_length_neutron is present in around 2500 entries out of over 530 000.