Modify simple examples based on the Optuna code conventions

[nabenabe0928]

Motivation

This PR refactors PR#216. Primary changes in this PR is to modify the codes so that the examples will follow the Optuna code conventions.

Description of the changes

The typical code conventions are the following:

1. Multi-objective function returns a tuple of objective values,
2. Make the comments clearer without the context, e.g. the first line in the main block of multi_objective/quadratic_simple.py should use more general nouns like "the first objective" rather than obj1, obj2, and
3. Specify the finished trial number if we use timeout.

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[toshihikoyanase]

flake8 found some coding-style violations in quadratic_simple_constraint.py. Could you fix them, please?

Run flake8 .
./quadratic_simple_constraint.py:5:100: E501 line too long (102 > 99 characters)
./quadratic_simple_constraint.py:6:100: E501 line too long (101 > 99 characters)
./quadratic_simple_constraint.py:8:100: E501 line too long (105 > 99 characters)
./quadratic_simple_constraint.py:10:100: E501 line too long (109 > 99 characters)
./quadratic_simple_constraint.py:[11](https://github.com/optuna/optuna-examples/actions/runs/6571297355/job/17850176657?pr=218#step:6:12):100: E501 line too long (114 > 99 characters)
./quadratic_simple_constraint.py:[13](https://github.com/optuna/optuna-examples/actions/runs/6571297355/job/17850176657?pr=218#step:6:14):47: W291 trailing whitespace
./quadratic_simple_constraint.py:30:100: E501 line too long (108 > 99 characters)
6     E501 line too long (102 > 99 characters)
1     W291 trailing whitespace

[nabenabe0928]

flake8 found some coding-style violations in quadratic_simple_constraint.py. Could you fix them, please?

Run flake8 .
./quadratic_simple_constraint.py:5:100: E501 line too long (102 > 99 characters)
./quadratic_simple_constraint.py:6:100: E501 line too long (101 > 99 characters)
./quadratic_simple_constraint.py:8:100: E501 line too long (105 > 99 characters)
./quadratic_simple_constraint.py:10:100: E501 line too long (109 > 99 characters)
./quadratic_simple_constraint.py:[11](https://github.com/optuna/optuna-examples/actions/runs/6571297355/job/17850176657?pr=218#step:6:12):100: E501 line too long (114 > 99 characters)
./quadratic_simple_constraint.py:[13](https://github.com/optuna/optuna-examples/actions/runs/6571297355/job/17850176657?pr=218#step:6:14):47: W291 trailing whitespace
./quadratic_simple_constraint.py:30:100: E501 line too long (108 > 99 characters)
6     E501 line too long (102 > 99 characters)
1     W291 trailing whitespace

Thank you! Done!

[not522]

@Alnusjaponica Could you review this PR?

[toshihikoyanase]

@Alnusjaponica Thank you for your thorough review. I have no further comments, so please merge after your review.