Rename CLI command analyze to suggest and refactor method names

[osma]

This PR changes the analyze command to suggest and the analyzedir command to index. Many methods, variable names, docstrings, comments and even some module names within the codebase are renamed.

This is mainly a terminological change, to avoid the word analyze which hereafter is only used for the preprocessing performed by Analyzers, and to introduce the word suggest when referring to the full subject indexing operation where subjects are suggested for a given document.

Fixes #267

[codecov]

Codecov Report

Merging #270 into master will increase coverage by <.01%.
The diff coverage is 99.41%.

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
+ Coverage   99.31%   99.31%   +<.01%     
==========================================
  Files          52       52              
  Lines        2629     2630       +1     
==========================================
+ Hits         2611     2612       +1     
  Misses         18       18

Impacted Files	Coverage Δ
annif/backend/ensemble.py	100% <100%> (ø)	⬆️
tests/test_backend.py	100% <100%> (ø)	⬆️
tests/test_cli.py	100% <100%> (ø)	⬆️
annif/project.py	100% <100%> (ø)	⬆️
annif/suggestion.py	99.02% <100%> (ø)
annif/backend/http.py	100% <100%> (ø)	⬆️
tests/test_project.py	100% <100%> (ø)	⬆️
annif/util.py	97.22% <100%> (ø)	⬆️
tests/test_eval.py	100% <100%> (ø)	⬆️
annif/backend/pav.py	98.52% <100%> (ø)	⬆️
... and 15 more

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