Refactor and fix MolecularContainer.get_pi()

[speleo3]

• Add test for pI (isoelectric point)
• Use bisection instead of grid search in MolecularContainer.get_pi(), which is much faster (probably about 10x)
• Also, the grid search did not produce the expected results for one of my input files. The "iteration" part of the grid search was apparently not entered and produced pI to a single digit precision only (9.0 instead of 8.65).

[codecov[bot]]

Codecov Report

Merging #148 (80c7bf0) into master (ab80128) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #148   +/-   ##
=======================================
  Coverage   73.09%   73.09%           
=======================================
  Files          25       25           
  Lines        3970     3970           
=======================================
  Hits         2902     2902           
  Misses       1068     1068           

Impacted Files	Coverage Δ
propka/molecular_container.py	92.59% <100.00%> (ø)

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