proper tool use

[GreenDirkfried]

For the case invulnerable_slade = true
I suggest to remove the groups = {cracky=3} for slade nodes.

Reasons:

• Slade nodes have a strange behavior, because you can dig them even with low level pickaxes very fast, before they are "placed back by the server". Which is confusing, because slade nodes should behave as tough material.
• This behavior allows for entering through slade if there is a weak connection from the client to the server.

For the case
invulnerable_slade = false
I suggest to treat different slade nodes differently (and tougher than most of the other MTG nodes), like

• the "df_underworld_items:slade_sand" having groups = {crumbly = 3, level=2}
• the "df_underworld_items:slade_brick" having groups = {cracky=1, level=2}
• the "df_underworld_items:slade_block" having groups = {cracky=1, level=3} (or even groups = {cracky=1, level=5})
• and so on...