change default inputs

[federicazanca]

          I'll write something in the documentation to exaplain how it works at the moment and open another issue to fix some things with the output in the future

_Originally posted by @federicazanca in https://github.com/stfc/aiida-mlip/pull/104#discussion_r1576439705_

[federicazanca]

some things need changing in the inputs for the calculations:

• [ ] at the moment we are using janus default values but we don't want to rely on them so redefine the default values from the inputs as None and then set a default if they are not defined in the config file
• [ ] the filenames are the only things that don't have a default at the moment, probably they will need changing and being handled the same way as the other inputs in a way that works in aiida (align the names among calcjob, parser etc)
• [ ] some parameters in the inputs/config file might change after some janus changes