Optional fdata and pdata in MSnSet constructor

[const-ae]

Sometimes I would like to quickly create a test MSnSet. From my understanding currently the standard MSnSet constructor requires the user to provide an fdata and pdata data.frame. I think it would be great if empty data.frame would created internally, if fdata or pdata is missing

suppressPackageStartupMessages(library(MSnbase))

mat <- matrix(rnorm(10 * 6), nrow=10, ncol=6)
rownames(mat) <- paste0("protein_", 1:10)
colnames(mat) <- paste0("sample_", 1:6)

MSnSet(mat)
#> Error in MSnSet(mat): argument "fData" is missing, with no default

SummarizedExperiment::SummarizedExperiment(mat)
#> class: SummarizedExperiment 
#> dim: 10 6 
#> metadata(0):
#> assays(1): ''
#> rownames(10): protein_1 protein_2 ... protein_9 protein_10
#> rowData names(0):
#> colnames(6): sample_1 sample_2 ... sample_5 sample_6
#> colData names(0):

^{Created on 2018-12-07 by the reprex package (v0.2.1)}

If you think it wouldn't be too difficult, I could try to make a PR.