Add human mouse mix reference v3.1.0

Changes needed:

• astrocyte_pkg.yml
• code has to be: GRCh38-and-mm10-3.1.0
• text should be 'Human GRCh38 + Mouse GRCm38 (mm10) release 93'
• put it directly above the two other human mouse mixes
• NOTE: please fix mm19 to mm10
• biohpc.config
• In params:genomes: add the code from astrocyte_pkg.yml translation to physical location than Paniz will hopefully be making for us (see slack message)
• README.md & index.md

changed milestone to %v1.2.0

changed the description

added enhancement label

marked the checklist item astrocyte_pkg.yml as completed

marked the checklist item biohpc.config as completed

marked the checklist item README.md & index.md as completed

mentioned in merge request !54 (merged)

closed via merge request !54 (merged)

mentioned in commit f30b5e5b