Add image with 64-bit int PETSc

[Prof Garth Wells]

We need 64-bit int PETSc for the test systems. To avoid a proliferation of images, this could be installed next to the 32-bit int PETSc, or maybe alongside Trilinos.

[Jack Hale]

I'd like to keep the dev-env image and stable size down, so we should put this in with Trilinos. It can then be the obscure linear algebra backend image ;o).

[Prof Garth Wells]

I think we should add 64-bit PETSc to the main image so that we can add it to the test framework without much complication. It only takes up ~40MB (less than the 32-bit version because there are fewer PETSc add-on packages that are 64-bit int compatible).

[Prof Garth Wells]

Build 64-bit PETSc and SLEPc. Fixes #21.

→ <<cset 560b9f5b5f9f>>