Add full support for (fetch|with)_assay_covariates to FacileData API

[lianos]

Currently the assay_sample_info_tbl holds "libsize" and "normfactor" columns which make sense for RNA-seq-like count data, however we can't store other types of assay information on the sample level anywhere.

One way to fix this is to make a new assay_sample_covariate EAV table where we can add arbitrary covariates to the samples, like the sample_covariate table.

Once we do this, we'd remove the "libsize" and "normfactor" columns from assay_sample_info_tbl and put them as long covariates in the assay_sample_covariate table.

We would then remove the fetch_sample_statistics faciledata API function.

• Create the assay_sample_covariate EAV table in the faciledata sql schema
• Move libsize and normfactor for dataset,sample_id,assay_name combinations into that table
• drop "libsize" and "normfactor" from assay_sample_info_tbl
• remove fetch_sample_statistics from FacileData API