Dem particle weight

[OGaboriault]

Description

Default value for the particle weight was outdated. This PR is changing this default value from 10 000 to 1000.
Some test output had to be changed because of this.

Miscellaneous (will be removed when merged)

Checklist (will be removed when merged)

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• All in-code documentation related to this PR is up to date (Doxygen format)
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• Lethe documentation is up to date
• The branch is rebased onto master
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent

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[blaisb]

2k seems like a legit default value. Let's use that.

[blaisb]

@OGaboriault can you rebase the changelog then I can merge?

[OGaboriault]

Done