Fix bug of top-k threshold task

[amallia]

Fixes #425.

Changes proposed in this pull request:

• clear top-k heap after every query execution

[codecov]

Codecov Report

Merging #426 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
- Coverage   92.51%   92.49%   -0.03%     
==========================================
  Files          91       91              
  Lines        4932     4932              
==========================================
- Hits         4563     4562       -1     
- Misses        369      370       +1     

Impacted Files	Coverage Δ
include/pisa/bit_vector.hpp	97.54% <0.00%> (-0.41%)	⬇️

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