Umpire allocator run-time choice

[bendudson]

When Umpire is used to allocate memory in Arraythe allocator is fixed to "pinned". This means that arrays are kept in host memory, and transferred to the device every time it's needed. Other allocators might lead to better performance on GPUs.

Umpire allows allocators to be registered, so a run-time option could configure and register in Umpire a "BOUT" allocator, which is then used in Array.