Old: first snapshot
2011-12-01: new push side wall function -- creates cytoskeleton forces
2012-04-10: Implemented Model3 for transpeptidase inhibition

when fail to build peptide crosslinking, glycan synthesis terminates,
effectively inserting a shorter glycan strand.
2012-04-12: Implemented a constriction force that is scaled by density

For each vertex at midcell, the froce is scaled by the total polygon
area associated with the vertex.
Note the old push_middle portion of the script is silenced within an if
(0) loop.
The idea is to differenciate the cases between same force for all
vertices and diff. force for vertices.
Should organize later.
Also silenced all the Align_Cell function,
which was implemented for pushing everything mimicking cytoskeletal
forces.
The implementation of cytoskeletal force needs improvement anyways,
as we've leanred via theoretical modeling.
2012-04-18: modified division force scaling

Added a feature to scale division force with k(1/Rc - 1/R).
At the same time, force is also scaled with sum of polygon area.
Altogether, we've put in all effects that I'd think FtsZ has on
division.

Take caution when using for other division force application.
The exclusion conditions are not laid out perfectly,
since there are overlapping features etc.
2012-04-25: Changed push_middle a bit such that the force drags towards origin
2012-05-05: improved push_middle function

This version has a vastly improved push_middle function
now, the origin is no longer the COM of the whole cell,
but rather the weighted center of the constriction area.
The conditional loops were also modified to make altenating between
force application modes easy.
Can now be done within the wall.in input file.
2012-05-16: modified force scaling

Corrected the wrong force expression for scale_force_with_R within the
push_middle division feature.
also added an alternative, linear force for scale_force_with_R.
a new parameter DivisionForceEqnForm is added to switch between the two.

The stuff above is commented out actually.
scale_force_with_R actually decrease the force.

A new FactorScalePolygonA is added to scale force with polygon area,
such that total force is actually not constant.
2012-05-24: reverted some push_middle behavior

division origin is 0 0 0, not measured from septum geometry
force direction is radially inward with x and y component; z component
is taken out

note (05-31-2012) turned off print lines for AngleThresh Theta values
This is for 93-LeightRandom, which we set AngleThresh to be 0 and we
don't want all those output lines
2012-06-04: Brought back scale constriction force by R
2012-06-08: modified division force scaling

The division force that's scaled by polygon area is now scaled inversely
by number of nearby vertices.
Number of nearby vertices is defined by a radius.
2012-06-12: modify constriction force calculation

For the part of the code that counts nearby vertices to determine
constriction force,
we're now using the width of constriction zone as radius of search.
also, count only the vertices within constriction zone
2012-07-11: peptide bonds with zero spring constants can now be added

Later (after 10 insertion events; has to change manucally), the springs
will get back to original kd.