Consistent file structure for quad

[mmahsereci]

In a Nutshell

No changes in functionality. No changes in docs. Simply making file structure consistent among quad modules.

Detailed Description

Should be reviewed after #742 is merged as it branches off of it.

Related Issues

Closes #...

[codecov]

Codecov Report

Merging #743 (e81d099) into main (77a2eef) will increase coverage by 0.02%.
The diff coverage is 98.19%.

@@            Coverage Diff             @@
##             main     #743      +/-   ##
==========================================
+ Coverage   90.48%   90.51%   +0.02%     
==========================================
  Files         199      202       +3     
  Lines        7517     7538      +21     
  Branches      974      974              
==========================================
+ Hits         6802     6823      +21     
  Misses        481      481              
  Partials      234      234              

Impacted Files	Coverage Δ
...rc/probnum/quad/solvers/policies/_random_policy.py	100.00% <ø> (ø)
...um/quad/solvers/belief_updates/_standard_update.py	95.55% <95.55%> (ø)
src/probnum/quad/integration_measures/__init__.py	100.00% <100.00%> (ø)
...num/quad/integration_measures/_gaussian_measure.py	100.00% <100.00%> (ø)
.../quad/integration_measures/_integration_measure.py	100.00% <100.00%> (ø)
...num/quad/integration_measures/_lebesgue_measure.py	100.00% <100.00%> (ø)
...rc/probnum/quad/solvers/belief_updates/__init__.py	100.00% <100.00%> (ø)
...bnum/quad/solvers/belief_updates/_belief_update.py	100.00% <100.00%> (+3.50%)	⬆️
...um/quad/solvers/policies/_van_der_corput_policy.py	100.00% <100.00%> (ø)

[JonathanWenger]

👍