refactor "inbreeding" Nushell modules (!141)

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
```bash
use consts.nu
use $consts.NUSHELL ...
```
- updates the README

bins/inbreeding/README.md

-- build the example for best performance with [`inbreeding build`](./build.nu)
-- run the experiment with [`inbreeding run`](./run.nu)
-- plot the results with [`inbreeding plot`](./plot.nu)
+- build the example for best performance with [`inbreeding build`](./src/.nushell/build.nu)
+- run the experiment with [`inbreeding run`](./src/.nushell/run.nu)
+- plot the results with [`inbreeding plot`](./src/.nushell/plot.nu)
 
 # Example
 ```bash
@@ -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)

bins/inbreeding/mod.nu

-export use build.nu
-export use watch.nu
-export use run.nu
-export use inspect.nu
-export use list.nu
-export use load.nu
-export use plot.nu
+export use src/.nushell/build.nu
+export use src/.nushell/watch.nu
+export use src/.nushell/run.nu
+export use src/.nushell/inspect.nu
+export use src/.nushell/list.nu
+export use src/.nushell/load.nu
+export use src/.nushell/plot.nu

bins/inbreeding/consts.nu → bins/inbreeding/src/.nushell/consts.nu

 export const BIN = "./target/release/inbreeding"
 export const CACHE = ($nu.home-path | path join .cache komodo inbreeding)
+
+export const NUSHELL = "../../../../.nushell"

bins/inbreeding/load.nu → bins/inbreeding/src/.nushell/load.nu

 use consts.nu
 use parse.nu [ "parse arg-experiment", "parse experiment" ]
 use path.nu [ "remove-cache-prefix" ]
-use ../../.nushell error "error throw"
+use $consts.NUSHELL error "error throw"
 
 use list.nu
 

bins/inbreeding/plot.nu → bins/inbreeding/src/.nushell/plot.nu

 use std repeat
 
 use consts.nu
-use ../../.nushell plot gplt
-use ../../.nushell color *
-use ../../.nushell error "error throw"
+use $consts.NUSHELL plot gplt
+use $consts.NUSHELL color *
+use $consts.NUSHELL error "error throw"
 
 def "parse strategy" []: string -> record<type: string> {
     let s = $in

bins/inbreeding/run.nu → bins/inbreeding/src/.nushell/run.nu

 use consts.nu
-use ../../.nushell error "error throw"
+use $consts.NUSHELL error "error throw"
 
 const VALID_HEX_CHARS = "abcdefABCDEF0123456789"