Fermions: apply signflips and efficient negation of blocks

[manuschneider]

The new methods apply() and apply_() exectute the signflips on the blocks.

Elsewhere, taking the negative of a block is replaced where possible by flipping the signflip metadata. This avoids overhead, since the sign is likely to be changed later anyways again.

[codecov]

Codecov Report

❌ Patch coverage is 49.01961% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.35%. Comparing base (e839fa8) to head (e87c3a2).
⚠️ Report is 18 commits behind head on master.

Files with missing lines	Patch %	Lines
include/UniTensor.hpp	41.17%	10 Missing ⚠️
src/BlockFermionicUniTensor.cpp	69.23%	7 Missing and 1 partial ⚠️
src/UniTensor_base.cpp	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
+ Coverage   32.16%   32.35%   +0.18%     
==========================================
  Files         215      215              
  Lines       36316    36363      +47     
  Branches    14583    14597      +14     
==========================================
+ Hits        11680    11764      +84     
+ Misses      22700    22659      -41     
- Partials     1936     1940       +4     

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