Write a new allocator for coreneuron

[ohm314]

To replace the more C-style alloc_memory, calloc_memory, emalloc_align and ecalloc_align with C++ class construction and allocation (using new and delete) we need to provide a new allocator that handles:

• default allocation (fallback to default allocators)
• aligned allocation
• allocation on the GPU

[ohm314]

This was partially taken care of here: https://github.com/BlueBrain/CoreNeuron/commit/557f2566427908b68918403da87e3af2af65914c But ultimately this will be part of our data-structure rewrite. For now we can keep it open, though.

[olupton]

https://github.com/BlueBrain/CoreNeuron/pull/606 also added some relevant utilities.