Use pinned memory for GPU version

Description

Add a special allocation function in C that will allocate pinned memory when running on GPUs for the wavefunctions. This increases the transfer speed by more than a factor of 2.

This function could in the future also be used to make the allocated memory aligned for CPU runs which would improve the usage of vectorization.

News snippet

Use pinned memory for the GPU version to improve transfer speeds.

Checklist

• I have checked that my code follows the Octopus coding standards

Closes #223 (closed). Relates to #162 (closed).