Allow for custom sample_id and gene_name column names in the melt_gene_exprs_mat function

[tinyheero]

Description

The current melt_gene_exprs_mat automatically specifies the sample_id and gene_name columns following melting/transformation into a long form. We should allow for users to input a custom column name.

For example:

melt_gene_exprs_mat(
  exprs_mat, 
  melt_row_name = "melted_gene_name", 
  melt_col_name = "melted_sample_id",
  melt_value_name = "melted_sample_id"
) 

This could produce a data.frame with column names like:

melted_gene_name	melted_sample_id	melted_sample_id

Tests

An automated test should be added to ensure that the columns match up with what was used as input.