Reduce tolerance for Chebyshev expansion to 1e-10

Description

Tests have shown that charge conservation is not good enough at a tolerance of 1e-5. For a methane example, the timestep can be chosen much larger (10-15x) compared to the Taylor example with only small changes in the resulting absorption spectrum. In total, a timestep is about twice as fast for this configuration.

News snippet

Reduce tolerance for Chebyshev expansion of the exponential to 1e-10

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.