RedistributeGeometryDecorator: axial and spherical powerlaw

[arlauwer]

Description
Added an abstract RedistributeGeometryDecorator that redistributes density distributions by multiplying it by a weight function. Currently there are 2 concrete sub-classes implementing a spherical and axial power law weight function.

Motivation
This decorator allows for adding gradients to already implemented geometries, which would otherwise have no analytical inverse cumulative. Such as for example hyperboloids and paraboloids that are not constant.

Testing
Testing involved comparing:

• spherical shell with power law p + a spherical decorator with power law q
• shell geometry with a power law of p+q
  Additionally, analytical values were compared with SKIRT-generated values for density and images to validate the decorator's functionality.

Context
More RedistributeGeometryDecorator can be added if needed. There is also a spherical and axial exponential weight function, but these have not been included.

[petercamps]

Good work. I introduced a few minor changes.

Most importantly, when a configurable property is "irrelevant" its value should never be used. This is why I added the extra tests in line 32 of AxPowerLawRedistributeGeometryDecorator.

Also, by convention, we add a . at the end of floating point constants such a 0. and 1. even though the compiler will almost certainly perform the conversion from integer to double at compile time.