JRH proposals

The current master branch contains a definition which changes the meaning of already defined tags (_refln_F_squared_calc). This is not permissible, even though the dictionary merging protocol allows it, as it destroys backwards compatibility of any programs that assume the immutability of tag meaning. For example, a program may retrieve _refln_F_squared_calc not realising that it no longer refers to a unique h,k,l and erroneously calculate Fourier maps.

Therefore, we must define a new category to allow listing of these values. As a side effect, this allows h,k,l, F_calc to be listed with their usual meanings - but F_obs cannot be included in such a list as the individual h,k,l no longer necessarily correspond to a single peak.