Enable large grids

Description

Use 8-byte integers throughout the code for global array sizes and indices. This enables grids with more than about a billion points - the barrier was around (2^31-1)/3 (about 700 million) before. I have tested large grids for the following cases:

1. Si with 1500^3 points (~3.3 billion points):

• a few SCF iterations
• a few TD steps

2. C allelectron run (~2.1 billion points, fft grid 3375^3)

• a few SCF iterations
• a few TD steps

3. Maxwell runs with a few timesteps

• about 1430^3 = 2.9 billion points
• about 2000^3 = 8 billion points

4. Al spin-polarized (~2.6 billion points, fft grid 3465^3)

• a few SCF iterations
• a few TD steps

5. Al spinor (~2.6 billion points, fft grid 3465^3)

• a few SCF iterations
• a few TD steps

Important points:

• these runs need a lot of memory
• 64bit versions of METIS and ParMETIS required for more than 2 billion points
• pfft is very useful for the Poisson solver, otherwise the memory requirements are even larger (normal Poisson solvers need a few global functions extra on each core)
• use only Pardomains for TD runs
• no LCAO, also needs a lot of memory
• output takes long, should be kept to minimum
• output size is big (>600 GB of restart data alone for Si 1500^3)

News snippet

Enable large grids with more than about 1 billion points

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.