circumvent finalize error

Dear Laurent,

as already mentioned in one of my last emails I always get the following error using my branch https://github.com/sgibb/MSnbase/tree/addIdentificationData :

    s <- readMSData(c("Thermo_Hela_PRTC_1_MS2cent.mzML","Thermo_Hela_PRTC_2_MS2cent.mzML"))
    Reading 13193 MS2 spectra from file Thermo_Hela_PRTC_1_MS2cent.mzML
      |=====================================================================================| 100%
    Reading 13265 MS2 spectra from file Thermo_Hela_PRTC_2_MS2cent.mzML
      |=====================================================================================| 100%
    Error in (function (x)  : attempt to apply non-function
    Caching...
    Creating 'MSnExp' object

    traceback()
    1: (function (x)
      x$.self$finalize())(<environment>)

It is very crazy because the error is thrown but R doesn't stop. The `MSnExp`
object is created correctly, e.g.:

    validObject(s)
    [1] TRUE

    length(spectra(s))
    [1] 26458

    i <- addIdentificationData(s, c("Thermo_Hela_PRTC_1_MS2cent.mzid","Thermo_Hela_PRTC_2_MS2cent.mzid"))
    dim(fData(i))
    [1] 26458    26

The `master` branch (`MSnbase == 1.11.10`) does not show this error.

Just for documentation: a similar error was described for Reference Classes by Martin Morgan in 2011 
https://stat.ethz.ch/pipermail/r-devel/2011-December/062773.html

Via `git bisect` I tried to find the commit which introduced the error. I run `git bisect` twice and get the following commit both times: https://github.com/sgibb/MSnbase/commit/22358bdb1d471fb494f5f25f4225a5604b78fb2a
I am total confused because this commit has nothing to do with `readMSData` and in fact I did not changed/touched `readMSData.R` at all.

Using R's debug function I could identify the following line throwing the error (https://github.com/lgatto/MSnbase/blob/dea3344b76c92dbd3e3e1580d4090ce26f786e3a/R/readMSData.R#L247):
```s
fl <- sapply(assaydata, fromFile)
```

I do not understand why this error occurs. According to https://stat.ethz.ch/pipermail/r-devel/2011-December/062773.html it has something to do with the `methods` package (and ReferenceClasses which are not involved in this line). That's why I decided to access the slot `fromFile` directly and avoid the use of the `fromFile,Spectrum-method`. And that fixes the error for me.

As I already mentioned I do not understand why the error occurs and I am not sure that my workaround is reasonable.

Could you reproduce the error using my `addIdentificationData` branch? What is your opinion about this workaround?

Best wishes,

Sebastian