* Two factors contribute to the above solution:
   - New eigen branch (version 3.5 and above). Currently we are
      using a mirrored version of Eigen in BlueBrain organisation
      https://github.com/BlueBrain/eigen/releases/tag/v3.5-alpha
   - An API that makes possible to call any Eigen `__device__` 
      function from within OpenACC regions.
* More details: Eigen-3.5+ provides better GPU support; however,
   some functions cannot be called directly from within OpenACC regions.
   Therefore, we need to wrap them in a special API (decorate them with 
    `__device__` & `acc routine` tokens), which allows us to eventually call
    them from OpenACC. Calling these functions from CUDA kernels presents
    no issue.
*  From #726: Avoid use `[]` operator with eigen Matrix objects. This results
    into runtime error with OpenACC and PGI compiler.
* Note that this should works in combination with BlueBrain/CoreNeuron/pull/624

fixes #311 
fixes #135