Support writing data matrices with target scale type without generating new vectors

[arteymix]

It should be possible to write data to disk and perform basic conversion between scale types.

This is relevant for single-cell data because we might not have control over the type of data we import.

[arteymix]

Ok, I'm done with all the possible conversions.

I'm still wondering if we should round data for producing counts...

[arteymix]

I think rounding is fair if we include a warning because there's always the option of passing LINEAR to get the adjusted counts.

[arteymix]

Ok, this is completed for single-cell data. It also needs to be applied to bulk matrix writers as well.