`zeroWeightsLS()` errors out if DESeq2 package isn't attached

[mjsteinbaugh]

The zeroWeightsLS() function errors out if DESeq2 isn't attached because the package internally doesn't locate DESeqDataSetFromMatrix() in the namespace correctly:

> weights <- zingeR::zeroWeightsLS(
+     counts = counts,
+     design = design,
+     maxit = maxit,
+     normalization = "DESeq2_poscounts",
+     colData = colData,
+     designFormula = designFormula
+ )
converting counts to integer mode
Error in DESeqDataSetFromMatrix(counts$counts, colData = colData, design = designFormula) : 
  could not find function "DESeqDataSetFromMatrix"
>