Add nn_ensemble backend

[osma]

Neural network ensemble backend based on Keras and TensorFlow. WIP.

Creating the (draft) PR early on so that I get feedback from the QA tools etc.

Fixes #330

[lgtm-com]

This pull request introduces 4 alerts when merging 0cf82c5 into 6ac8431 - view on LGTM.com

new alerts:

• 3 for Unused import
• 1 for Module is imported with 'import' and 'import from'

[codecov]

Codecov Report

Merging #331 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #331      +/-   ##
==========================================
+ Coverage   99.51%   99.53%   +0.01%     
==========================================
  Files          55       57       +2     
  Lines        3080     3203     +123     
==========================================
+ Hits         3065     3188     +123     
  Misses         15       15

Impacted Files	Coverage Δ
tests/test_backend_nn_ensemble.py	100% <100%> (ø)
annif/util.py	97.36% <100%> (+0.14%)	⬆️
annif/backend/__init__.py	100% <100%> (ø)	⬆️
annif/backend/nn_ensemble.py	100% <100%> (ø)

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[osma]

TODO items:

• make (hyper)parameters configurable (should be coordinated with Issue273 default values for configuration settings #324 / Default values for configuration settings #273, probably that work should be merged first)
• decide if this should remain an optional extra, or a core dependency (keras + tensorflow)
• pin versions of keras and tensorflow; see if we can get rid of deprecation warnings by choosing compatible versions
• document this backend in the wiki

[osma]

TensorFlow 2.0.0 is out, I think it would make sense to switch to that instead of relying on 1.15.0rc1 which will be the last 1.x release anyway.

[lgtm-com]

This pull request introduces 1 alert when merging f12929a into 8e90e24 - view on LGTM.com

new alerts:

• 1 for Syntax error

[osma]

Any comments @juhoinkinen or @mvsjober to the Keras model defined here? Would you do something differently?

[juhoinkinen]

Looks good

[lgtm-com]

This pull request introduces 1 alert when merging b9093e7 into 8ca56f4 - view on LGTM.com

new alerts:

• 1 for Module-level cyclic import

[lgtm-com]

This pull request introduces 1 alert when merging f0014df into 8ca56f4 - view on LGTM.com

new alerts:

• 1 for Module-level cyclic import

[lgtm-com]

This pull request introduces 1 alert when merging ce49174 into 8ca56f4 - view on LGTM.com

new alerts:

• 1 for Module-level cyclic import