Backport tool charge helpers

set_charge/get_charge from other Technic mod

Battery box technic_get_charge / technic_set_charge

Mineunit rock

Update mineunit (#133)

* Switch to mineunit for regression tests

* Update mineunit

Co-authored-by: SX <sx@minetest>

Fix digtron HV connector again (broken in #113) (#121)

Co-authored-by: SX <sx@minetest>

update contributors