fix a small bug of writing CMTSOLUTION file

[lynlyf]

What does this PR do?

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Why was it initiated? Any relevant Issues?

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PR Checklist

• Correct base branch selected? master for new features, maintenance_... for bug fixes
• This PR is not directly related to an existing issue (which has no PR yet).
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  Just remove the space in the following string after the + sign: "+ DOCS"
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• All tests still pass.
• Any new features or fixed regressions are be covered via new tests.
• Any new or changed features have are fully documented.
• Significant changes have been added to CHANGELOG.txt .
• [*] First time contributors have added your name to CONTRIBUTORS.txt .

[ThomasLecocq]

hi, thanks for the PR. Please:

• provide context of why it is needed
• provide an explanation about the choices of setting 0 instead of 1 for the STF
• correct the contributors file, you didn't put your name at the right location (the one you modified links to the right file)

[lynlyf]

Hi Thomas,

I'm a bit confused. I apologize in advance if it's due to my mis-operations to pull all the changes (I always work locally with github and am not so familiar with the inter-repositories PRs).

I only requested a PR long time ago to fix the bug of writing the magnitudes in the CMTSOLUTION file [the commits on Jan 6, 2021]. The other changes were only made in my own repository. I changed the missing half duration values from 1.0 to 0.0 to distinguish missing values from existing values (over 100 events in the CMT catalog already have their half duration equal to 1.0). I'm not sure if other obspy users need such a utility.