Implement exponential midpoint rule for classical particles/celestial bodies

Description

Implement a new propagator, the exponential midpoint rule, for the classical particles (aka celestial bodies). This approach uses a version of the classical equations of motion rewritten as a Schrödinger-like equation and then discretizes the time evolution operator with the midpoint rule.

The exponential is computed using a Taylor expansion of order 4.

For the tests, the values for the 2 body and 3 body problem (same timesteps) have been generated with the python code to validate the algorithm independently. These validated values have only been adapted for the tests with different timesteps and/or propagators.

News snippet

Add exponential midpoint rule for classical particles.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Closes #349 (closed) #357 (closed)