Calculate isoelectric points of polypeptides using methods of Bjellqvist.

pK values and the methos are taken from::

* Bjellqvist, B.,Hughes, G.J., Pasquali, Ch., Paquet, N., Ravier, F., Sanchez,
J.-Ch., Frutiger, S. & Hochstrasser, D.F.
The focusing positions of polypeptides in immobilized pH gradients can be predicted
from their amino acid sequences. Electrophoresis 1993, 14, 1023-1031.

* Bjellqvist, B., Basse, B., Olsen, E. and Celis, J.E.
Reference points for comparisons of two-dimensional maps of proteins from
different human cell types defined in a pH scale where isoelectric points correlate
with polypeptide compositions. Electrophoresis 1994, 15, 529-539.

I designed the algorithm according to a note by David L. Tabb, available at: http://fields.scripps.edu/DTASelect/20010710-pI-Algorithm.pdf