MadGraphControl: fixes and options for MadSpin running

Mike Hancerequested to merge
mhance/athena:21.6-madspin-MGC-patches into 21.6

Includes a fix for finding LHE files after running madspin. Option to revert LHE event weights after madspin modifies them in spinmode=none. See e.g. https://answers.launchpad.net/mg5amcnlo/+question/684169. Disabled by default.
Tagging @zmarshal and @hmildner from an email thread.

Edited by Tadej Novak

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