Multigoal linter

[jstoobysmith]

The whole of PhysLean needs the MultiGoal linter run on it and all the warnings dealt with.

In particular

[leanOptions]
linter.style.multiGoal = true

should be added to lakefile.toml and the project built with lake build.
This will generate warnings, some of which may be from Mathlib, but the ones from PhysLean should be dealt. For example:

   exact h 
   exact h2 

should be replaced with:

  · exact h1 
  · exact h2