Avoid unnecessary calculation of eigenvectors in EigVals::eigvals and relax the DataMut bound

[jturner314]

This PR does the following:

• Changes EigVals::eigvals to not calculate the eigenvectors.
• Relaxes the bound on EigVals from DataMut to Data.
• Adds calls to .eigvals() in the tests.

[codecov]

Codecov Report

Merging #286 (484cabe) into master (a561e5a) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #286      +/-   ##
==========================================
+ Coverage   89.01%   89.31%   +0.29%     
==========================================
  Files          71       71              
  Lines        3577     3593      +16     
==========================================
+ Hits         3184     3209      +25     
+ Misses        393      384       -9     

Impacted Files	Coverage Δ
ndarray-linalg/src/eig.rs	100.00% <100.00%> (+33.33%)	⬆️
ndarray-linalg/tests/eig.rs	100.00% <100.00%> (ø)
lax/src/eig.rs	94.00% <0.00%> (+5.00%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a561e5a...484cabe. Read the comment docs.

[termoshtt]

This fixes #255