CST_Anomaly doesn't allow to use obs or exp only

As noticed by @eida, CST_Anomaly doesn't allow to compute only anomalies for obs or exp.

It seems that the problem comes from the initial check: if (!inherits(exp, 's2dv_cube') || !inherits(obs, 's2dv_cube')) {

because it is not considered the case that one of them could be NULL.