Sparse arrays with NAs

[lgatto]

The example below speaks for itself:

> library(scp)
> data("leduc_minimal", package = "scp")
> library(SparseArray)
> NaArray(assay(leduc_minimal))
<200 x 73 NaMatrix> of type "double" [nnacount=7182 (49%)]:
                  eAL00219RI5  eAL00219RI6 ... wAL00286RI17 wAL00286RI18
       SAVEDEGLK           NA           NA   .     11.01346          NaN
        APNVVVTR     13.56997     13.85389   .     11.80130     11.45907
       IVVVTAGVR           NA           NA   .     12.68021     12.82554
  GFQEVVTPNIFNSR     10.78733      9.36700   .          NaN          NaN
  QLNNLALLCQNQGK           NA           NA   .           NA           NA
             ...            .            .   .            .            .
      LTDQVMQNPR    10.292322     9.723388   .           NA           NA
      LGAEVYHTLK     9.423704     8.523444   .           NA           NA
 AANSLEAFIFETQDK          NaN          NaN   .          NaN          NaN
       FLLAVSRDR           NA           NA   .     10.34729     10.51096
EASMVITESPAALQLR           NA           NA   .           NA           NA

Ping @cvanderaa