making quad iterator info stateless & adding tests

[mmahsereci]

In a Nutshell

This PR makes the following changes in quad:

• Separates better the BQIterInfo and BQState: The former is now simply a container for info concerning the loop iteration. The latter contains the BQ belief.
• BQIterInfo is now a simple dataclass and also stateless (it wasn't stateless before, and error prone due to inconsistent updating).
• Tests added for both BQIterInfo and BQState as there were no tests at all before.
• Some docstring fixes, mostly removing colons as discussed in Long-term global `pylint` ignores #470 (comment)

Detailed Description

This PR will be followed up by changes as in #620 .

Related Issues

Closes #...

[codecov]

Codecov Report

Merging #659 (ce55cb8) into main (bc699dd) will decrease coverage by 0.00%.
The diff coverage is 97.87%.

@@            Coverage Diff             @@
##             main     #659      +/-   ##
==========================================
- Coverage   90.02%   90.01%   -0.01%     
==========================================
  Files         194      194              
  Lines        7337     7343       +6     
  Branches     1159     1159              
==========================================
+ Hits         6605     6610       +5     
  Misses        489      489              
- Partials      243      244       +1     

Impacted Files	Coverage Δ
src/probnum/quad/__init__.py	100.00% <ø> (ø)
src/probnum/quad/solvers/policies/_policy.py	100.00% <ø> (ø)
...rc/probnum/quad/solvers/policies/_random_policy.py	100.00% <ø> (ø)
src/probnum/quad/solvers/bayesian_quadrature.py	86.04% <93.33%> (-0.71%)	⬇️
src/probnum/quad/_bayesquad.py	94.44% <100.00%> (ø)
src/probnum/quad/solvers/__init__.py	100.00% <100.00%> (ø)
src/probnum/quad/solvers/bq_state.py	100.00% <100.00%> (ø)
...olvers/stopping_criteria/_bq_stopping_criterion.py	100.00% <100.00%> (ø)
.../quad/solvers/stopping_criteria/_immediate_stop.py	100.00% <100.00%> (ø)
...olvers/stopping_criteria/_integral_variance_tol.py	100.00% <100.00%> (ø)
... and 2 more