Suppress loading/plotting of event lists

[willend]

Free-form text area

Please describe what your PR is adding in terms of features or bugfixes:

Suppress loading/plotting of event lists
(fixes #292 / #1150 / #1245)

McStas 3.6.15 plot with event data:

This PR with suppressed event data:

As a bonus, failure to load an uncompleted / broken file will lead to an empty plot with the text 'Unknown format'

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Development OS / boundary conditions

Please describe what OS you developed and tested your additions on, and if any special dependencies are required:

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PR Checklist for contributing to McStas/McXtrace

For a coherent and useful contribution to McStas/McXtrace, please fill in relevant parts of the checklist:

• My PR is meant to fix a specific, existing issue

  • I have indicated the issue number here: mccode-r.c: Define special # type for event lists #292 / Enhancement of Monitor_nD list-mode and its graphics-outputs #1150 / Suppress plotting of list mode data #1245
  • I have added documentation for the fix and possible side effects

• My contribution contains something else

  • Explanation is added in free form text above or below the checklist

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