[operators] Improve subtraction of LincombOperators

[sdrave]

Fixes #479.

[codecov]

Codecov Report

Merging #480 into master will increase coverage by 0.43%.
The diff coverage is 62.5%.

Impacted Files	Coverage Δ
src/pymor/operators/basic.py	63.27% <60%> (-0.52%)	⬇️
src/pymor/operators/constructions.py	83.85% <63.15%> (-0.21%)	⬇️
src/pymor/core/pickle.py	85.71% <0%> (-5.72%)	⬇️
src/pymor/algorithms/to_matrix.py	100% <0%> (ø)	⬆️
src/pymor/algorithms/gram_schmidt.py	98.33% <0%> (+0.46%)	⬆️
src/pymor/operators/block.py	68.42% <0%> (+2.37%)	⬆️
src/pymor/core/config.py	92.47% <0%> (+3.43%)	⬆️
src/pymor/reductors/basic.py	89.23% <0%> (+4.23%)	⬆️
src/pymor/bindings/scipy.py	78.75% <0%> (+4.75%)	⬆️
src/pymor/tools/table.py	94.44% <0%> (+4.78%)	⬆️
... and 1 more