Replace eigen buffer

[ryanmrichard]

Is this pull request associated with an issue(s)?
No.

Description
After #232 MDBuffer should be a drop-in replacement for buffer::Eigen. The goal of this PR is to move TensorWrapper closer to the spirit of the original design where the buffer objects are backend-agnostic. To this end, this PR will:

• Delete buffer::Eigen (and the associated infrastructure)
• Rename MDBuffer to Contiguous (see TODOs for caveats)
• Delete the allocator component and replace it with free functions make_contiguous_buffer (names subject to change given above caveat).
• floating_point_dispatch is now the responsibility of WTF not TensorWrapper.

TODOs

• Hook-up commented out features (were commented out to get this to compile).
• Get tests to work again.
• Reexamine the design to see if Contiguous is the right name. In particular, where does a non-contiguous buffer fit? What about a hierarchical buffer?
• Ensure patches to downstream repos are ready.