Disc patch revived

Implements the changes discussed in #311. 

 - The disc patch example is now 400x400x800 in length with correct internal energy.
 - The accelerations get truncated at |z-z_disc| > 300 with a cosine transition
 - The accelerations are 0 at |z-z_disc| > 380.
 - The calculation is significantly faster thanks to the extraction of constant terms.

See merge request !349