Batch suggest in SVC backend

[osma]

This PR implements _suggest_batch instead of single-document _suggest in the SVC backend, allowing the use of more efficient vector operations.

I tested this with the 20 Newsgroups data set example, as shown on the wiki page of the SVC backend. Evaluation on the test set is 20-30% faster with the results otherwise unchanged.

There is one special case that requires attention: what to do when the input to the SVC classifier is empty or near-empty. The old _suggest code handled this as a special case so it short-circuits the classifier and returns an empty result. There is also a unit test that tests this (using the special input text "j" which is an unknown token and thus equivalent to empty input). The SVC model has no problem returning some class (maybe the majority class?) in this case, but I reimplemented the special case check for empty input to retain the old behaviour that refuses to classify it and keeps the unit test happy.

With 1 job

	user time	wall time	max rss
before (master)	20.36	0:20.20	237848
after (PR)	14.35	0:14.17	237332

With 4 jobs

	user time	wall time	max rss
before (master)	23.16	0:07.92	186972
after (PR)	15.41	0:06.02	187756

Fixes #667

[sonarcloud]

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0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Base: 99.56% // Head: 99.56% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (1c587bd) compared to base (cc6dfcf).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #670      +/-   ##
==========================================
- Coverage   99.56%   99.56%   -0.01%     
==========================================
  Files          87       87              
  Lines        6145     6142       -3     
==========================================
- Hits         6118     6115       -3     
  Misses         27       27              

Impacted Files	Coverage Δ
annif/backend/svc.py	100.00% <100.00%> (ø)

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