Externals/GPU Update, master branch (2020.05.18.)

This is a replacement for !31709 (closed), which I'll just close.

The MR updates all projects to use atlasexternals-2.0.68, which brings the following updates since atlasexternals-2.0.67 (atlasexternals@2.0.67...2.0.68):

• Updated Acts to version v0.22.1 (ping @pagessin);
• Updated GeoModelIO to version 3.1.3 (ping @rbianchi and @mbandier);
• Taught AtlasCMake how to deal with .cu source files when CUDA is not available for the build;
• Introduced the triSYCL external, to provide SYCL for builds done without a "SYCL capable" compiler.

To account for the latter two updates, I also made a few changes in this project:

• Updated the AthExCUDA package to build differently in case CUDA is/isn't available. Meaning that it will now build functional code without CUDA as well.
• Updated the AthExSYCL package to build successfully against triSYCL as well.
  • Since FindSYCL.cmake is now provided by atlas/atlasexternals, removed that module from AthExSYCL.
• Updated the Athena and WorkDir projects to use CMake's check_language(...) for detecting which languages they should set up for their build. Instead of hardcoding for WorkDir which base projects require Fortran, and which don't...

Pinging @leggett, @fwinkl and @baines. 😉