vw-multi backend: use input from other projects

[osma]

Makes it possible to specify what inputs are used to build the VW model. Thus it's possible to use e.g. a well trained subject indexing algorithm as input to the vw classifier. #230

[codecov]

Codecov Report

Merging #256 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
- Coverage   99.31%   99.28%   -0.03%     
==========================================
  Files          51       51              
  Lines        2466     2503      +37     
==========================================
+ Hits         2449     2485      +36     
- Misses         17       18       +1

Impacted Files	Coverage Δ
tests/test_backend_vw_multi.py	100% <100%> (ø)	⬆️
annif/backend/fasttext.py	98.63% <100%> (+0.03%)	⬆️
annif/backend/vw_multi.py	100% <100%> (ø)	⬆️
annif/backend/mixins.py	94.11% <100%> (-5.89%)	⬇️

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[kinow]

Not with my IDE in this computer, so couldn't check the code suggestions 😢 feel free to ignore and just proceed with the merge 👍

[osma]

Thanks for all the suggestions @kinow ! I think this is much better now, and good enough for merging.

[kinow]

Agreed! And thank you for the interesting pull requests!

As you are sending pull requests instead of commiting directly, that is giving me some time to review and familiarise with what's coming in the next release.

Plus, easier to get.notifications and manage them ☺

[osma]

I like doing PRs against my own project because this way I can trigger all the CI and static analysis tools and see their result in one place. It's almost like having another person review your code as you're writing it, although now you did that for real :)