Update HCA preview data gene counting matrix

[LuyiTian]

Gene counting matrix generated by scPipe

Thank you for contributing to easy-data! Please make sure your markdown file explaining the dataset has the following:

• a description of the dataset including a link to the appropriate publication or
  reference.
• direct links to download the count matrix (in the form of an easy-to-load file, like a rds file containing an sparse matrix for R and an AnnData hdf5 file or
  a mtx file for python).
• direct links to download the metadata (in a csv with rows indexed by cell names).
• sample loading code for R and python.

[jacksonloper]

Thanks for putting this together. Looks like we don't actually have direct links on this though (i.e. a URL that actually points directly to something you download, not to page on which you can download things). Could you flesh it out, more in the fashion of https://github.com/czi-hca-comp-tools/easy-data/blob/master/datasets/tabula_muris.md?

[batson]

lgtm

[batson]

@jacksonloper any additional changes you would make?

[jacksonloper]

Sorry I'm still confused :(. The count files seem to be to the Ischaemic data, but then the metadata seems to be to the Melanoma data. Would it be possible to clear up exactly what data you are preseting from the HCA preview page? As far as I can tell there are three studies there. Which is this supposed to represent?

We also want a reference (ideally even a link!) to the relevant publication, e.g. to

Census of Immune Cells - Contributors: Bo Li, Monika S Kowalczyk, Danielle Dionne, Orr Ashenberg, Marcin Tabaka, Timothy Tickle, Jane Lee, Karthik Shekhar, Michal Slyper, Julia Waldman, Orit Rozenblatt-Rosen, Aviv Regev. Collaborating organisations: Broad Institute, Massachusetts Institute of Technology, Howard Hughes Medical Institute.

Sorry to be so much trouble :).