Disc patch revived

Implements the changes discussed in #311 (closed).

• 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.

examples/DiscPatch/HydroStatic/README

@@ -18,3 +18,5 @@ output to 'Disc-Patch-dynamic.hdf5'. These are now the ICs for the actual test.
 
 When running SWIFT with the parameters from 'disc-patch.yml' and an
 ideal gas EoS on these ICs the disc should stay in equilibrium.
+
+The solution can be checked using the 'plotSolution.py' script.