Support ndarray 0.15

[cassiersg]

Adding std feature is required since ndarray 0.15 made
impl std::error::Error for ShapeError require std.

[cassiersg]

@termoshtt If this is wanted, an update to rust 1.49.0 of the rustmath/mkl-rust would be needed (1.49 is the MSRV of ndarray 0.15).

[codecov]

Codecov Report

Merging #273 (fbac1a9) into master (28c7860) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head fbac1a9 differs from pull request most recent head 06af362. Consider uploading reports for the commit 06af362 to get more accurate results

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
- Coverage   89.04%   88.95%   -0.09%     
==========================================
  Files          71       71              
  Lines        3586     3557      -29     
==========================================
- Hits         3193     3164      -29     
  Misses        393      393              

Impacted Files	Coverage Δ
ndarray-linalg/src/cholesky.rs	97.14% <ø> (-0.05%)	⬇️
ndarray-linalg/src/eigh.rs	91.83% <ø> (+1.64%)	⬆️
ndarray-linalg/src/lobpcg/lobpcg.rs	92.12% <ø> (-0.82%)	⬇️
ndarray-linalg/src/lobpcg/svd.rs	85.36% <ø> (ø)
ndarray-linalg/src/solve.rs	72.58% <ø> (-0.08%)	⬇️
ndarray-linalg/src/solveh.rs	73.73% <ø> (+0.47%)	⬆️
ndarray-linalg/src/lobpcg/eig.rs	75.38% <100.00%> (ø)
ndarray-linalg/tests/cholesky.rs	100.00% <100.00%> (+4.08%)	⬆️
ndarray-linalg/src/operator.rs	85.00% <0.00%> (-5.00%)	⬇️
ndarray-linalg/src/layout.rs	85.29% <0.00%> (-2.95%)	⬇️
... and 19 more

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[termoshtt]

Merged with #277, thanks