Octopus 15

• Update Spack package (switch to CMake, missing pfft support in Octopus' CMake; we stick to autotools for the global octopus@15)
• Install Octopus 15 as global package
• Drop Octopus 13 (?)

changed milestone to %24b

assigned to @martin.lang

marked the checklist item Drop Octopus 13 (?) as completed

marked the checklist item Update Spack package (switch to CMake) as completed

changed the description

marked the checklist item Install Octopus 15 as global package as completed

changed milestone to %25a

mentioned in merge request !136 (merged)

mentioned in commit fa57dd05

closed with merge request !136 (merged)