Export a gene information table

[dhimmel]

Similar to how we have sample information in samples.tsv, it would be nice to create a table with gene information. The primary identifier is entrez_gene_id. Additional columns could be:

• symbol
• name
• chromosome
• n_mutations - number of mutated samples
• median_expression - median gene expression
• mad_expression - median absolute deviation of gene expression

I'm leaning towards a combined dataset for mutation and expression genes. But I could be convinced that splitting the datasets would be better.

We should probably get this information from entrez gene as @clairemcleod did in #12.

Labeling this issue a task awaiting a claimer.

[cgreene]

@dhimmel median_expression and mad_expression explanations might need a quick edit.