- ProfileColumn.HHblitsNullModel as extra fixed null model
- ProfileHandle.AddColumn method now adds a character to the sequence
- ProfileHandle has a default null model (HHblits) always set
- ProfileHandle attributes sequence and null_model are now read-write
  (replaced SetSequence, SetNullModel)

this pops up as warning when compiling with a super new gcc (6.2)

!!EXPERIMENTAL FEATURE!! The plugin gets loaded by OpenMM as a dynamic
library and is capable of updating forces acting on particles in a
running OpenMM simulation/minimization. The DensityForce takes
a ost map as an input and generates forces based on

Topf et al., 2008 Protein Structure Fitting and Refinement Guided by CryoEM Density

The feature needs to be redesigned for mmCIF files since chain names can have
more than 1 character. This could be added at a later point in time to speed up
loading large mmCIF files, if only a part is needed.

Up to 20 string comparisons had to be performed in the previous implementation.
This number has been reduced by using a switch statement on the first letter
of the residue name.