Pull request type

• Code changes (bugfix, features)

Checklist

• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

Rocketpy SolidMotor class simulates the grain regression either radially and axially by default.

This PR is related to the following issue:
ENH: Enable only radial burning #801

New behavior

Now it's possible to the user to change from the default behavior to a only radial regression by setting the new "only_radial_burning" optional parameter as True in the SolidMotor class.

Breaking change

• No

Additional information

Here is an example of the comparison between the default burning and the only radial one applied to the getting_started.ipynb simulation. The pytest tests haven't passed locally because of a windows problem.