Add no.missing argument to bcScore

[mj-jimenez]

Currently, bcScore takes an argument expr.thres that determines the minimum proportion of genes that must be expressed per each cell and signature pair in order to compute a BCS. For those uncomputed scores, bcScore returns NaN. However, it might be interesting to avoid non-missing values (for regression, for example). As a normalized BCS = 0 denotes that we do not know the enrichment direction, I think NaN could be replaced by 0 if no.missing = TRUE. Default value for no.missing should be FALSE to not alter current behavior.