WIP: update CUDA branch

Update and clean up the cuda branch to be based on an up-to-date version of master with minimal changes.

TODO:

• update to version 1.1
• clean up code syntax
• fix open issues to make it work with modern CUDA/GPUs
• update to version 1.5.2 (commit 111567ee)
• update to version 20.1.0 (commit cd10d71e)
• update to the latest version
• minimise changes from master
• test and verify that it still works

Note: to build the 1.1 version (commit ec984838), one needs to first apply the changes in commit 761cba64.