divide by 0 in M3D_normalized

```
mist.getM3D_normalized <- function(x) {
    # divide cols by their bait length
    x1 <- x[, 5:dim(x)[2]]/x[, 3]
```

scaling by 0 or NA lengths introduces Inf/NA values in normalized matrix. We should check for those and optionally remove them while throwing warnings.
