Keep demux_cell_count_estimate name for output metadata file

[avrohomgottlieb]

Context

We currently rename demux_cell_count_estimate to sample_cell_estimate when we're collecting Sample metadata in Sample::get_metadata. The decision was made to no longer rename this attribute for the output metadata file.

As well, we would like to assign this attribute's value in the output metadata file to be its corresponding value in the sample_cell_estimates field of the Library metadata file (which we have access to in Library.metadata). We don't plan on changing the way that the demux_sample_cell_estimate attribute is calculated in Project::update_sample_derived_properties, just the value in the output metadata file. This should be done in Library::get_metadata.