Keep track of the number of cells with at least one expressed gene in samples and pseudo-bulks

[arteymix]

This is important to know how many cells contribute to a given pseudo-bulk or sample.

It is not equal to the number of cells per sample since we do not retain all the genes upon import.

• number of cells with at least one gene expressed
• number of genes with at least one cell expressing it
• number of non-zero cell-by-gene values

[arteymix]

We have an way of computing this for MEX. It might be better to use the pre-computed value once this is implemented.

[arteymix]

For MEX, were actually computing the NNZ in the matrix, which is different.

We should also consider keeping that number and also the number of genes with at least one cell expressing it in a sample.

[arteymix]

If we are going to do this, we need to validate that there are no actual zeroes in the sparse data vector.

Also, for log-transformed data, zero is -inf, this might be something we have to deal with.

[arteymix]

Ok, the code is there, I just need to implement the logic for filling the fields.