Code not working for multiple particle objects

[SachinJalan]

Steps to generate the issue.

1. Generate an example with a single cell and a single particle object and run the solver.
2. Generate an example with exactly same configuration except have 2 particle objects having same material properties and element_ids.
   The results does not match.
   Issue in the code:- In this line of the code scheme.py , compute_nodal_mass function will be called same number of times as the number of particle objects and in this function this line will set the nodal mass to 0 every time the function is called hence the nodal mass for the last called Particle object would remain. Similar issue with the compute_nodal_momentum function.

[chahak13]

Thanks for pointing this out. I also realised this and #27 should resolve this too.