NF: initial sketch for a script to enable NeuroDebian repo on any Debian

yet to be finalized... and maybe/hopefully supplied with the tests.  But seems to work and ultimately will be wrapped up with a debconf interface of a "neurodebian" package to be uploaded to stock Debian ;)

So even now could be tried (dry run) e.g. via
```
bash <(wget -q --no-check-certificate -O- https://raw.githubusercontent.com/yarikoptic/neurodebian/enh/neurodebian-installer/tools/nd-aptenable) -v -v -n --suffix=-de-sid-full -c software,data,devel
```
TODO: fetch/use information about mirrors