Fix concatenate and add missing functionality to backends:

[kratsg]

Description

Add the following functionality to backends in preparation for tensorizing the interpolation code as in #251

• einsum
• zeros
• abs

NB: mxnet does not support einsum. In lieu of this, I added a raise NotImplementedError when einsum is called.

Checklist Before Requesting Approver

• Tests are passing
• "WIP" removed from the title of the pull request

[lukasheinrich]

for background:

this is part of the major rewrite to vectorize pyhf better (#231 and #251) are splitting it into multiple PRs. The vectorization of the interpolation code makes heavy use of the einstein summation-style tensor operations

[lukasheinrich]

we should add some tests for einsum and axis-aware concat

[coveralls]

Coverage decreased (-0.1%) to 97.27% when pulling 64f3ac2 on update/addFunctionalityInBackend into 75edf5b on master.

[matthewfeickert]

Yeah, I agree. This all looks really nice (:+1:), so once tests are in it should be ready to :shipit:.

[matthewfeickert]

LGTM (I see that the push passed the CI).

[lukasheinrich]

ok, i'll ignore the .1% decreasein coverage and merge