Remove requirement for mesh in peripy setup

[tjdodwell]

Currently runs are built from first defining a mesh - which is odd for a meshless method.

Define a simple standard input file for defining particles

[tjdodwell]

x, y, z, isConstrained_x, isConstrained_y, isConstrained_z, isForce_x, isForce_y, isForce_z, material_tag

[tjdodwell]

Also need to think about how we define a constitutive model of bonds in a purely particle-based model setup