BUG: Motor.export_eng doesn't work for liquid motors

[Gui-FernandesBR]

Describe the bug

Take a look the method's definition.
As the liquid motors do not have propellant grains, this method will fail.

To Reproduce

Create a liquid motor and try to export its data using export_eng method

Expected behavior

• We should refactor this method and ensure that it work for
• We should also create unit tests to prevent this behavior to happen again (please include all the types of motor in this test, even the GenericMotor)

Screenshots

[lucasfourier]

Working on it right now