Update gh-action for tests and codecov (#124)

set version tag to 0.26.1

Update __about__.py

Aktualisieren von __about__.py

Update __about__.py

Update _linalg_tensor.py

Merge pull request #113 from adtzlr/add-math-special-erf

Add the (Gauss) error function `math.special.erf(z)`

Support arrays in `math.special.try_stack()`

Merge pull request #111 from adtzlr/fix-eigh

Fix missing `eigh` dual values

Merge pull request #110 from adtzlr/add-sqrtm

Add matrix square root `math.linalg.sqrtm()`