Fix to MPI-mode use of Brilliance Transfer calculation.

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Free-form text area

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

This fixes an issue spotted by Cristy Kinane from ISIS where an instrument applying a brilliance-transfer calculation gave numbers that were systematically off.

To ensure proper operation across MPI and non-MPI settings we now use

• DArray1D types for the in-instrument BT monitor (underlying arrays are calloc’ed and hence ensured to contain zeros)
• Perform the array-operations on Master only
• Add an MPI_Barrier to not get out of sync

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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 contribution includes patches to an existing instrument file

  • I have used the mcdoc utility and rendered a reasonable documentation page for the instrument (please attach as screenshot in comments!)
  • I have used the mctest utility to test the instrument (please attach mcviewtest report as screenshot in comments)
  • I have used the mcrun --c-lint "linter" and followed advice to remove most / all warnings that are raised

• My contribution contains something else

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

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