Use triangular solves in default `_solve`

[timweiland]

In a Nutshell

Make use of triangular property in _solve

Detailed Description

• Make the default _solve implementation of LinearOperator use triangular solves if the linear operator is known to be lower/upper triangular
• Add test cases for triangular matrices

[codecov[bot]]

Codecov Report

Merging #801 (aa0b889) into main (4d435e4) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #801   +/-   ##
=======================================
  Coverage   91.28%   91.29%           
=======================================
  Files         218      218           
  Lines        8207     8209    +2     
  Branches     1059     1060    +1     
=======================================
+ Hits         7492     7494    +2     
  Misses        487      487           
  Partials      228      228           

Impacted Files	Coverage Δ
src/probnum/linops/_linear_operator.py	87.21% <100.00%> (+0.04%)	:arrow_up: