Unit Tests

Unit tests are needed to check:

Periodic boundary conditions:
- particle outside of first periodic box
- non-cubic boxes
- long bonds

Random walk:
- uniform thetas and phis
- all particles are given a position
- all bonded particles are a bond length away

DPD frame instance:
- A particles are added to the frame
- positions from random walk are added to the frame
- bonds, angles and dihedrals are added to the frame

DPD simulation:
- parameters match input from kwargs
- running in a loop, checking simulation end criteria
- random number seeds

Start by locating the implementations and existing test setup for periodic boundary conditions, the random walk, DPD frame construction, and DPD simulation. Run the current tests first, then cover the listed edge cases and parameter, loop, and seed behavior; done means each requested behavior has a passing unit test.

Written by the indexing model from the issue text.