Re #22315

Build was failing with ‘nan’ is not a member of ‘std’ on Ubuntu 18.04.

This is based on a reply from Freddie when I recently asked him about
this.

In order to allow LoadNexusMonitors to use it, they need to be made
static.

Made the check slightly more readable, and also more robust about
there being multiple script directories with the word Diffraction in

This code fails to compile with clang 6.0.0 because of the noexcept on
the defaulted constructor. For more info see this stack overflow isssue:
    https://stackoverflow.com/questions/46866686/default-member-initializer-needed-within-definition-of-enclosing-class-outside

Avoids a copy of the underlying string on gcc < 5 where std::string used
COW mechanism. Marking the string const forces the compiler to select
the const overload of string::operator[], which avoids the copy
and race condition in the OpenMP loop.

This applies to function style algorithm calls in Python via simpleapi.
Before, the default was to set EnableLogging=True regardless of parent
algorithm's setting.

Re #22403