Fix sign of longitudinal wake when reading from headtail file

[xbuffat]

Description

The sign convention of the longitudinal wake (positive means energy loss) is opposite to PyHT. The sign changed is adjusted in the conversion factor when reading the file.

Closes # .

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

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• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts