Allow twisting of trivial sectors without restrictions

[lkdvos]

Enable twisting of trivial sectors in cases where no braiding is defined, similar to how braiding trivial sectors is special cased.

Fixes #239

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.61%. Comparing base (ec7ee9d) to head (87301b5).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/tensors/indexmanipulations.jl	16.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
- Coverage   82.68%   82.61%   -0.08%     
==========================================
  Files          43       43              
  Lines        5573     5579       +6     
==========================================
+ Hits         4608     4609       +1     
- Misses        965      970       +5     

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[Jutho]

Looks good; feel free to merge.

Note: it would be good to have tests for both the braiding and twisting behaviour of indices with a unit sector at some point. However, we currently don't even have any non-group symmetric tensor tests.

[lkdvos]

MPSKit remains a somewhat decent test suite of this, since it's actually being used there :)

I was actually quite surprised to find that the trivial braiding thing worked, and had to dig through the code a bit to find out how this was being achieved.