Expand the library of built-in meshes

[ahellander]

As an intermediary step to having a more integrated way to set geometries and meshes, we should expand our library of built-in meshes. For example, we should have:

Spherical Cell Coarse
Spherical Cell Medium
Spherical Cell Fine

Spherical Cell with Membrane Coarse
...

Spherical Cell with Nucleus Coarse (this is the Hes1)
...

EColi Coarse
EColi Medium
EColi Fine

These should all be done with Gmsh, and in the UI in the mesh information, we should have a link to download the .geo file. In this way, it will be easy for users to modify/customize the models. It is a bit hard to learn the Gmsh BREP model of specifying geometry, but it is much easier to learn by modifying an existing file.