[Refactor] finish switch to real McCode sources

[g5t]

• By implementing CMake-like configurefile support for replacing `@MCCODE*@strings by newly-added configuration keys the remaining dependency on the pre-configuredmccode-r.h,mcstas-d.h, andmcxtrace-d.h` has been removed.
• The GitHub based registry files are still outside of the main McCode repository, but they are updated automatically nightly. In the future they may be updated by changes in the McCode repository.
• Merging this should finally fix #30

[g5t]

One reason to use the upstream sources: I changed the return type of _getcomp_index from int to long at some point, probably because the index of a component is long valued. That change was not propagated back to the main repository, so tests here fail when the 'real' sources are used.

@willend Can you confirm whether the component index property is long valued? And, assuming it is, would you accept a PR to change the _getcomp_index return from int to long?

[g5t]

The component _index property is indeed long valued. See https://github.com/McStasMcXtrace/McCode/blob/c4c9421d630767f612703947bad4c82941220398/mcstas/src/cogen.c.in#L542

[g5t]

McCode #1597 should track resolution of the type discrepancy. The last commit here may need to be undone as a result.