dgCMatrix is deprecated

Calling the exposure function with structural equivalence returns a warning:

```
exposure(kfam_diffnet,alt.graph ="se", valued =TRUE)
```

Returns

```
'as(<dgeMatrix>, "dgCMatrix")' is deprecated.Use 'as(., "CsparseMatrix")' instead.See help("Deprecated") and help("Matrix-deprecated").
```

This needs to be fixed by the next release.