[MRG] add codecov configuration to fix paths

[ctb]

This PR adds codecov.yml for configuration of our code coverage reporting at codecov.io so that the paths end up matching the source code paths.

Specifically, since we moved all the source code under src/, but the paths are being reported as sourmash/ instead of src/sourmash/ by our CI environment, codecov can't find the actual source code. This fixes that.

[codecov]

Codecov Report

Merging #1449 (8277399) into latest (29130d2) will increase coverage by 5.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1449      +/-   ##
==========================================
+ Coverage   89.45%   94.60%   +5.14%     
==========================================
  Files         123       96      -27     
  Lines       19120    15506    -3614     
  Branches     1472     1472              
==========================================
- Hits        17104    14669    -2435     
+ Misses       1784      605    -1179     
  Partials      232      232              

Flag	Coverage Δ
python	94.60% <ø> (ø)
rust	?

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Impacted Files	Coverage Δ
tests/test__minhash.py
src/core/tests/minhash.rs
src/sourmash/cli/lca/classify.py
src/sourmash/cli/index.py
src/sourmash/lca/__init__.py
src/sourmash/cli/compute.py
src/sourmash/command_sketch.py
src/core/src/sketch/nodegraph.rs
src/sourmash/lca/lca_utils.py
tests/test_sbt.py
... and 209 more

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[ctb]

I can't tell if this is working yet, I think because codecov isn't necessarily picking up the changed configuration while it's on this branch. @luizirber @bluegenes could I get approval to merge?

[luizirber]

Hmm, with the way tox is set up we are not really tracking the coverage of the src/ folder, but the installed sourmash package in the venv. For example, if you look at https://github.com/dib-lab/sourmash/runs/2317657067?check_suite_focus=true#step:12:368 you can see it is pointing to /home/runner/work/sourmash/sourmash/.tox/py39/lib/python3.9/site-packages/sourmash/__init__.py

That said, fixing the path with what you added in this PR fixes the problem, so LGTM! =P

[ctb]

Exactly. Worth a try, I figure :)

[luizirber]

Let's see what happens next time there is a Rust code change, because I think it is going to mess up the report

[ctb]

<khan> RUUUUUUUUUUUST! </khan>