Allow multiple runs of 'distancetospheres' method

[johannesring]

This pull request adds the ability to run the distancetospheres meshing method multiple times with distinct parameters for each run.

[johannesring]

This is useful for example if you want to refine the mesh in one region and then you want an even finer mesh at some critical points inside this region. For example, here I use 8 parameters for the distancetospheres meshing method:

vasp-generate-mesh -i C0002.vtp -m distancetospheres -mp 0 0.1 0.6 0.9 0 0.2 0.35 0.9

When running this, a window will pop up and I can place spheres where I want the finer mesh quality. The first four values given to the -mp option will be used, resulting in a mesh with edge length varying from 0.6 to 0.9. For example like this: After closing the Window, a new window will appear. The result from the previous step will be used. Now, the next four values will be used, and I can place new spheres, for example like this: The resulting mesh will look like this: Hope this help you to understand how to use this.

[keiyamamo]

@johannesring Thank you for the explanation! It is clear to me:)