new particle_shape_and_density

[tluettm]

concept for new handling of particle attributes depended on shape (area, volume, mass, capacity...)

• decoupled liquid and ice particles into different classes so that their shape can be changed separately
• currently, spherical shape of liquid particles is assumed implicitly in a lot of routines. Not a problem right now, but plenty of opportunity for refactoring in the (far) future!
• however, for ice particles all access to shape related variables should go through particle_shape_ice ~~common parent class of spheres for liquid and ice spheres~~ ~~I realize that the use of static methods in the spheres class and definition to self.mass density in the initiation of sub classes, liquid and ice spheres, does not make much sense in the current iteration~~
• is the access to the trivia functions implemented correctly by importing the entire trivia object?
• the proposed change would require to rework mixed moments routine

In the future, we could move to a logic where the shape is assigned (and can be changed!) for all super particles individually instead of separating them into just two (liquid and ice) particles. However, more 'fancy' shape realtionships like shape habit prediction could also be used with the current seperation intwo two types