Issue251 cli init command

[juhoinkinen]

The init command simply deletes all files under data/projects/project-to-remove/ (that directory will remain):

$ annif init tfidf-fi-copy 
Removed data files for project tfidf-fi-copy.

$ annif init nonexistent-project
No projects found with id 'nonexistent-project'.

Stated in the issue comment: "It could wipe all state and reset any configuration settings."
Is this PR enough for this?

The issue #269 for creating backend for using Maui server plans using init command for "creating tagger on the Maui server end", which is left for future.

This closes #251.

[codecov]

Codecov Report

Merging #306 into master will increase coverage by 19.55%.
The diff coverage is 95.23%.

@@             Coverage Diff             @@
##           master     #306       +/-   ##
===========================================
+ Coverage    79.8%   99.35%   +19.55%     
===========================================
  Files          55       55               
  Lines        2842     2962      +120     
===========================================
+ Hits         2268     2943      +675     
+ Misses        574       19      -555

Impacted Files	Coverage Δ
annif/cli.py	99.48% <100%> (+0.01%)	⬆️
tests/test_cli.py	100% <100%> (ø)	⬆️
annif/project.py	99.41% <83.33%> (+0.62%)	⬆️
annif/corpus/subject.py	100% <0%> (ø)	⬆️
annif/corpus/types.py	100% <0%> (ø)	⬆️
annif/corpus/document.py	100% <0%> (ø)	⬆️
annif/corpus/convert.py	100% <0%> (ø)	⬆️
tests/test_corpus.py	100% <0%> (ø)	⬆️
annif/analyzer/__init__.py	92.59% <0%> (+3.7%)	⬆️
... and 12 more

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[osma]

It's very good now but I'm still thinking about what the semantics should be here.

Originall I thought that this command could also be used to initialize Maui Server (once we add support for it) but I'm not so sure anymore - maybe that's overloading the command too much. It would be simpler to just define this command as clearing the state of a project - as is currently implemented. Consequently it would be best to call the command annif clear instead of annif init.

So just two things needed before this can be merged:

1. Rename the command to clear
2. Show a warning instead of an error if there is nothing to remove.

Sorry for dithering on this!

[osma]

I think this should emit a warning message but it shouldn't be a fatal error.

[juhoinkinen]

Implemented showing warning instead of erroring.

In the test for nonexistent data the command is run for dummy-fi project, because when running for a nonexistent project a (different) warning is raised already in get_project.

The test has logger.propagate = True for the same reason as in #299 (comment).

[osma]

Still need to document this on the wiki page Commands