refactor "inbreeding" Nushell modules

this MR

• moves the Nushell modules from bins/inbreeding/ to bins/inbreeding/src/.nushell/
• creates a NUSHELL constant in consts.nu to allow the following more robust construct

use consts.nu
use $consts.NUSHELL ...

• updates the README

bins/inbreeding/README.md

@@ -42,7 +42,7 @@ inbreeding load $experiment | inbreeding plot
 
 ## plot all experiments
 ```bash
-use bins/inbreeding/consts.nu CACHE
+use bins/inbreeding/src/.nushell/consts.nu CACHE
 use bins/inbreeding
 
 const FIGURES_DIR = ($CACHE | path join figures)