Science Spread

This library helps with spreading science packs out through the tech tree, while also trying to ensure that the tree can be researched. This means:

the mod won't add your packs to other sciences in a way that causes it to have no labs that can research it
ensures that every prerequisite is there.

To use it, register this prototype (only register it if this mod is there else the game will crash):
```lua
{
type= "science-spread-rule"
name = "<unique name>"
-- this pack won't be added unless these are also included on the technology already.
required_packs = {
"automation-science-pack",
"logistic-science-pack"
},
-- These technologies will use the starter tech as a dependency, and have your pack as an ingredient.
-- If some of them are not found (eg you have a krastorio technology in here and it is not installed) then it will be skipped safely.
-- You can just add all the technologies you want as a starter and it will be sorted out.
starter_techs = { <list of technology names that should start the spread of this pack. Any technologies using this tech as a parent will have your pack> },
pack_to_spread = "<name of the pack that should be spread across the tech tree>"
-- optional, every starter tech that was in the game will use this as a prerequisite.
pack_tech_name= "<name of the tech that spreads this technology, uses the pack's name if not present>"
}
```
For every technology you can also set the `tech_spread_exclude` field to true to make this mod not touch that technology.
Excluding a single pack is experimental, and not thoroughly tested.