Add function to voxelise a mesh

[p-j-smith]

Partially fixes #176

Changes made:

• Create a function (openep.mesh.mesh_routines.voxelise) to voxelise a surface mesh
• The function works by first constructing a series of surface meshes by interpolating points between the endocardium and epicardium and surfaces. Points from this series of meshes are then binned in a 3d histogram to determine which voxels should be filled
• There is an option to extract the myocardium (i.e. have a voxelised mesh that consists only of the myocardium), or the have a cuboidal mesh with labels used to distinguish empty of filled voxels
• A homogenous thickness is assumed, but thickness data (a value for each surface point) can be passed to create a mesh with heterogenous thickness