Write fcs function

[mbuttner]

Pytometry should include a function to export an anndata file to fcs using the Gating ML 2.0 version and FCS standard 3.*

[quentinblampey]

In scyan, there is this write_fcs function that we could move to pytometry
But it uses fcswrite internally, and it seems this repository is not actively maintained, what do you think?

EDIT: we can also add this write_csv function, since FlowJO supports reading from CSV files

[grst]

A couple points from the discussion in scverse/governance#64:

• There was some agreement that it makes sense to keep IO separate from the rest in readfcs. I'm sure they would be happy to include functions for writing FCS filese
• Here it could make sense to either switch to @whitews's FlowIO package or use it as another backend in readfcs.

adata = readfcs.read("sample.fcs", backend="fcsparser")
adata = readfcs.read("sample.fcs", backend="flowio")
readfcs.write(adata, "sample.fcs", backend="flowio")

• Or maybe to add support for AnnData directly in FlowIO?

pip install flowio[anndata]

adata = flowio.FlowData('sample.fcs').to_anndata()