Make electrons a child of system_t
Description
Make the electrons a child class of system_t. For the moment most of the methods are empty. They should be filled later. Also started to reorganize some of the code in td.F90 to make it easier to reuse it in the new framework.
News snippet
Started porting the electrons to the new multisystem framework.
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.