Many Small Improvements (memory leaks, build system, etc.)

This MR includes several small changes/fixes, which concisely are:

build system

• create .sac2crc directory by default; no longer bother user about this (we didn't do this initial so that users had a chance to switch from the file to directory form of sac2crc)
• remove unused (dead) feature flag ENABLE_SL
• move distmem configuration stuff behind feature flag (activated for CI)
• switch from CMake's FindCUDA to FindCUDAToolkit package (due to deprecation)

tests

• add new meta keyword REQFEATURES to set feature dependencies on SaC tests (e.g. if CUDA backend isn't available, don't run CUDA tests)
  • remove usage of isalloptionson.sh script in pogo-tests (@rbe)
• improve documentation

CI/masterrun

• add ubuntu 21 image

compiler

• update all copyright year text; add authors (😉)
• update bug tracker links to gitlab issue (oops, should have been a while ago)
• update uthash header to latest release
• error, instead of abort, on missing/wrong module name