Bug#961185: mumps: provide 64-bit build

[Drew Parsons]

Source: mumps
Severity: normal

We're discussing introducing a 64 bit-build for the computational
stack. This refers to 64-bit addressing to cells in meshes, etc.

c.f. Bug#953116 (petsc)
Bug#961108 (openmpi)
https://lists.debian.org/debian-science/2020/05/msg00051.html

BLAS is already 64-bit enabled.

For the 64-bit environment to work, it needs to be carried through all
along the library stack, from BLAS to PETSc.

MUMPS is in the middle.

Probably we want 2 separate builds, 32-bit (the current libmumps-dev)
and a separate 64-bit libmumps64-dev.

Opening this bug to track the chain of 64-bit package dependencies.