ELPA_2020.05.001_release

• Enable compilation with gcc v10
• Fix a bug in elpa_multiply_a_b (GPU)
• improved documentation, including fixing of typos and errors in markdown
• Fix a bug in the calling of Cannons algorithm which might lead to crashes
  for a squared process grid
• improvements and bugfixes of the ELPA2 stage GPU version, see
  https://arxiv.org/abs/2002.10991
• bugfix for the build of AVX-512 KNL kernels
• clean seperation of SIMD instructions for AVX and AVX2 kernels
• better error checking for allocations / deallocations of CPU and GPU memory
• experimental feature of matrix redistribution
• bugfix in the cpuid tests
• bugfix in elpa2_print_kernels
• bugfix when configuring --with-gpu-support-only