Improve support for large meshes

Added

• Text files can be used to set geometry positions/radii/etc from the command line.
• New obstruction type: BendyCylinder. This cylinder can follow an arbitrary path (defined using splines) and vary in diameter.
• 3D plotting for meshes (and BendyCylinder) is now available. Just call plot(mesh) or plot_geometry3d(mesh).

Changed

• Multiple values passed on the mcmr geometry on the command line should now be separated by commas and semi-colons rather than spaces
• The save_memory option has been removed from meshes, because it did not work properly.

Fixed

• Creation of large meshes has been sped up (and no longer leads to StackOverflow errors for very large meshes).
• Off-resonance fields generated from meshes are now correctly modelled using magnetic dipoles rather than magnetic monopoles.
• Size calculation of meshes has been fixed (used in the calculation of the maximum timestep).
• Determining the inside of meshes has been fixed.
• Fixed printing of arrays of Snapshot.