updated the test file and optimized Sternheimer
Description
Updated the test for the electron-photon time-propagation to include more than one mode. Also, further optimized the electron-photon Sternheimer part and added parts in the file that reads in the photon mode from external file.
Checklist
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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 #480 (closed)