better handling of unknown symmetry representations

[lxvm]

This pr makes sure that the correct result is returned for integrands that have unknown symmetry representations by returning a result computed with FBZ. This is a slow-down but we should always aim to be correct. Some of the heuristics that scalar integrands have a trivial representation may be incorrect if that scalar is a matrix element of some non-trivial operator, but requiring declaring a symmetry representation would be breaking.

[codecov]

Codecov Report

Attention: Patch coverage is 70.90909% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 86.57%. Comparing base (6b8c8df) to head (c02b526).

Files	Patch %	Lines
src/brillouin.jl	66.66%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
- Coverage   87.29%   86.57%   -0.73%     
==========================================
  Files          15       15              
  Lines        1220     1266      +46     
==========================================
+ Hits         1065     1096      +31     
- Misses        155      170      +15     

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