Make scalar * Zero = Zero

[timweiland]

In a Nutshell

Previously, multiplying the Zero linear operator by a scalar would return a ScaledLinearOperator. But we can just directly return the original Zero linear operator.

Why?

This simplifies composed linear operators a little bit, particularly because the Zero linear operator can now be absorbed further, e.g. in a sum. Consider the following snippet:

>>> from probnum.linops import Zero, Identity
>>> i = Identity((20, 20))
>>> z = Zero((20, 20))
>>> i + 3 * z
SumLinearOperator [
	<Identity with shape=(20, 20) and dtype=float64>,
	3.0 * <Zero with shape=(20, 20) and dtype=float64>
]

With this change, the result is

>>> i + 3 * z
<Identity with shape=(20, 20) and dtype=float64>

[codecov]

Codecov Report

Merging #817 (636c850) into main (2b93645) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #817   +/-   ##
=======================================
  Coverage   91.14%   91.15%           
=======================================
  Files         218      218           
  Lines        8210     8216    +6     
  Branches     1061     1061           
=======================================
+ Hits         7483     7489    +6     
  Misses        499      499           
  Partials      228      228           

Impacted Files	Coverage Δ
src/probnum/linops/_arithmetic.py	94.20% <100.00%> (+0.17%)	⬆️