estimate_size_factors with poscounts fails for dgCMatrix

Hi Constantin,
since there is no `colMedians()` method for `dgeMatrix` objects, I get the following error from `MatrixGenerics`
```
Error in MatrixGenerics:::.load_next_suggested_package_to_search(x) :
Failed to find a colMedians() method for dgeMatrix objects.
```

Casting to `matrix` somewhere around
https://github.com/const-ae/transformGamPoi/blob/d408b11e728fa44805a52ad9aa059b54f6b195a4/R/helpers.R#L126-L131
should fix the issue.

Best,
Christoph

Reproducible example:
```
set.seed(42)
counts <- scales::squish(x = rpois(n = 200, lambda = 3) - 3, range = c(0, Inf))
counts <- Matrix::Matrix(data = counts, nrow = 10, ncol = 20)
sf <- transformGamPoi:::estimate_size_factors(Y = counts, method = 'poscounts')
```