Faster WeightedSampler implementation

[dvolgyes]

Np.sortedsearch performs binary search, which is logN steps, instead of
the arithmetics on the full array and then a linear search (argmax).

[dvolgyes]

I meant np.searchsorted.

[codecov]

Codecov Report

Merging #308 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #308   +/-   ##
=======================================
  Coverage   92.89%   92.89%           
=======================================
  Files         109      109           
  Lines        5070     5070           
=======================================
  Hits         4710     4710           
  Misses        360      360           

Impacted Files	Coverage Δ
torchio/data/sampler/weighted.py	90.10% <100.00%> (ø)

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[fepegar]

I was about to check this PR. Did you just push to the wrong branch?

[dvolgyes]

Yes, but hopefully i undid it.

[fepegar]

Added in v0.17.40. Thanks, @dvolgyes!