Draft: Fix build on DAS-6

Several changes to CMake were needed to compile the code on DAS-6. 1. Completely remove the variants files 2. Fix compilation errors due to missing OpenMP symbols 3. Fix CMake warning regarding missing extension in source filenames 4. Remove compilation of some pyparameterset and pytools code

Regarding 1): imo., variants files are an outdated concept that prevent the code to be compiled on any machine that doesn't have a variants file. In 2023, CMake is more than capable enough to find dependencies on its own (or with the help of the FindXXX.cmake files). On DAS-6, many dependencies reside in a custom prefix, but CMake had no problem detecting them. Hence, a variants file was not needed. For reference, these were the modules that were used:

module load cmake
module load gcc/9.4.0
module load cuda
module load casacore
module load fftw
module load log4cplus
module load pqxx/6.4.8
module load postgresql
module load boost
module load dal2
module load openmpi
module load unittest-cpp
module load hdf5
module load binutils
module load readline
module load openblas
module load numactl

Note that only pqxx requires a fixed (old) version, others packages worked with the versions that we happened to have installed.

I am not sure how you were able to compile the code, given the issues encountered for 2) and 4). Finally, 3) is just a minor clean-up.