add SIp and NSAF

Dear Laurent,

this PR adds SIp and NSAF but there is no docmentation or test cases yet.

Usage:
```s
f <- list.files("../../playground/label_free", pattern="6000.mzML", full.names=TRUE)
id <- list.files("../../playground/label_free", pattern="6000.mzid", full.names=TRUE)

s <- readMSData(f)
si <- addIdentificationData(s, id)

mset <- MSnbase:::quantifySI_MSnExp(si)
SI(si, method="SIp")
SAF(si, method="NSAF")
```

Best wishes,

Sebastian