Optimize second step of LUniv

Use List.forall instead of Set.--
Time reduction up to 10x
Removed unoptimized LUniv
Removed NodeKind type
Many optimization tries for LUniv
Cleanup LUniv after optimization
BUGFIX: don't lower if the dual is in Delta

If Delta contains both a literal and its dual, the proof after deletion
is inconsistant. This might result in proof bigger than with LU (4%) or
proof with wrong conclusion (0.5%).
Alternative (non-working) LUniv algorithm
BUGFIX: valent literal must be in the conclusion
BUGFIX: valent literals must be in the conclusion
Merge remote branch 'origin/feature/fastLUniv' into fastLUniv

Conflicts:
	src/main/scala/at/logic/skeptik/algorithm/compressor/combinedRPILU/LowerUnivalents.scala
Bump version
Fix compiler warnings
Merge branch 'fastLUniv'
Rename subfolder of "examples/proofs/"
Merge commit '5e9a0ed9f68b6d8958205418cabf822517a79255' into release/1.0

* commit '5e9a0ed9f68b6d8958205418cabf822517a79255':
  BUGFIX: valent literals must be in the conclusion
  BUGFIX: valent literal must be in the conclusion
  Alternative (non-working) LUniv algorithm
  BUGFIX: don't lower if the dual is in Delta
  Cleanup LUniv after optimization
  Many optimization tries for LUniv
  Removed NodeKind type
  Removed unoptimized LUniv
  Optimize second step of LUniv

Conflicts:
	src/main/scala/at/logic/skeptik/util/help.scala
Add instructions on how to import Skeptik as a library
Add usage example and minor improvements in the README
Fix markdown typo
Merge branch 'release/1.0' into develop