Image based heterogeneous tissue

[frisograce]

Please describe your 🚀 feature request.
The ability to add heterogeneity to the tissue from a pre-specified image

Describe the solution you'd like
Implement a new class in heterogeneity_generator.py to allow for heterogeneity to come from a pre-specified image

[jgroehl]

You should be able to just add any correctly sized numpy array into the volume fraction of any of the molecules/structures.
Is the idea to have a class that reads a (e.g. PNG) image and converts it into a numpy array?
The class should probably automatically scale the input image to the 3D volume in a clever way (customisable by flags)?

[frisograce]

The merge request just added shows just uses numpy arrays for the time being, but functionality for images is certainly possible for the future!

Indeed the idea is that the class will scale the images in multiple ways as wished by the user (I've already added quite a few but more are possible) and have used tags to help the user navigate them!