Improve code coverage

[FedericoStra]

The code coverage is currently abysmally low (55% at the time of writing).

In particular:

• the files

  • src/diffusion.jl
  • src/piecewise_constant_functions.jl
  • src/plotting.jl

  are completely untested;

• the files

  • src/interactions.jl
  • src/models.jl

  are below 35%.

[FedericoStra]

#16 covers 100% of src/piecewise_constant_functions.jl.