symath structure change

Instead of use newton as general model,
symath implements a generalisation for all sub-implementations: the descriptors.

A descriptor is an abstract structure of which update behaviour can be implemented
in the sub-structures,
for example a NewtonDescriptor contains:

• mass
• position 3D
• speed 3D
• sum of forces 3D
• update integrates the derivates

Particle become a graphics class which implements a physics.

A Particle provides a pointer to a symath descriptor to be allocated in
the subclasses.

A NewtonParticle has been added in order to implement a Particle which uses
a NewtonDescriptor.

ParticleSystem improved.

Extending render of a Shape is simpler.