nvcc does not like duplicate arguments, here it failed with
    nvcc fatal   : redefinition of argument 'std'

Refactoring the Dirichlet and the Neumann boundary conditions.

Adding simple heat equation benchmark for CUDA.

(I've actually encountered such error...)

The problem is that for example Arch Linux symlinks python to python3,
which has different syntax (e.g. print is a function instead of a
statement).