featureCV ignores its na.rm argument

[sgibb]

While creating the MSnSets for the synapter objects I recognized NAs in the CV column. The NAs even survive na.rm=TRUE:

MWE:

library("MSnbase")
data(msnset)
msnset <- msnset[41:43]
featureCV(msnset, factor(rep(1, nrow(msnset))), na.rm=FALSE)
#   CV.iTRAQ4.114 CV.iTRAQ4.115 CV.iTRAQ4.116 CV.iTRAQ4.117
#1     0.4695498     0.2907382            NA     0.5489767
featureCV(msnset, factor(rep(1, nrow(msnset))), na.rm=TRUE)
#   CV.iTRAQ4.114 CV.iTRAQ4.115 CV.iTRAQ4.116 CV.iTRAQ4.117
#1     0.4695498     0.2907382            NA     0.5489767

expected output:

featureCV(msnset, factor(rep(1, nrow(msnset))), na.rm=TRUE)
#   CV.iTRAQ4.114 CV.iTRAQ4.115 CV.iTRAQ4.116 CV.iTRAQ4.117
#1     0.4695498     0.2907382     0.4233534     0.5489767

I have rewritten the featureCV to fix this bug and to speed up the calculation a little bit (because of less loops).

I opened this PR because there are some failing unit tests (because my pRolocdata is too old) but I can't download a newer version now. Maybe you could have a look and run the tests.