Use large integers in external qshep package

Description

This is needed for interpolations. As we do some of them on the global grid, they need to be able to handle large grids with 8-byte integer indices. This commit converts the full qshep package to 8-byte integers. Where 4-byte integers are passed in the code, they are converted to 8-byte.

Checklist

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