LinOp arithmetic part 1

[schmidtjonathan]

In a Nutshell

Add (efficient) arithmetics for combinations of Linear Operators : Part 1

Detailed Description

For some combinations of linear operators, there exist known arithmetics that can be implemented more efficiently.
E.g. Kronecker @ Kronecker ~~> Kronecker instead of yielding a ProductLinearOperator that builds up large call stacks.

Related Issues

#509 ; mitigates the described problem in parts. (Subsequent PRs will consecutively solve the issue)

[codecov]

Codecov Report

Merging #505 (ac2c5e0) into main (2e4153c) will increase coverage by 0.57%.
The diff coverage is 87.77%.

@@            Coverage Diff             @@
##             main     #505      +/-   ##
==========================================
+ Coverage   88.49%   89.06%   +0.57%     
==========================================
  Files         175      176       +1     
  Lines        6301     6612     +311     
  Branches      805      881      +76     
==========================================
+ Hits         5576     5889     +313     
+ Misses        501      485      -16     
- Partials      224      238      +14     

Impacted Files	Coverage Δ
src/probnum/_config.py	100.00% <ø> (ø)
src/probnum/randvars/_constant.py	86.00% <ø> (ø)
...probnum/randprocs/markov/integrator/_integrator.py	89.18% <33.33%> (-10.82%)	⬇️
src/probnum/randvars/_normal.py	81.77% <50.00%> (+0.08%)	⬆️
src/probnum/linops/_scaling.py	82.05% <71.42%> (+0.23%)	⬆️
src/probnum/linops/_linear_operator.py	83.22% <76.36%> (+0.82%)	⬆️
src/probnum/linops/_kronecker.py	86.89% <89.47%> (+0.89%)	⬆️
src/probnum/linops/_arithmetic_fallbacks.py	92.85% <92.85%> (ø)
src/probnum/linops/_arithmetic.py	92.38% <94.01%> (+22.21%)	⬆️
src/probnum/linops/__init__.py	100.00% <100.00%> (ø)
... and 10 more

[marvinpfoertner]

Nice! Let's discuss some details on monday.

Since we have Zeros now, could you also add the

...
elif self._scalar == 0:
    ...

case in the constructor of Scaling?

[marvinpfoertner]

The PR can't be merged because patch coverage is slightly too low.
From my POV we can merge this anyway, since test coverage did improve significantly in the linop modules.
@JonathanWenger @pnkraemer can you merge this?