Fix issues raised by golosio

Created by: AlexVanMeegen

Various fixes for the issues raised by @golosio in #21 (closed) and #22 (closed) and some additional minor fixes:

• add pytest to the requirements
• fix the SIM_LABELS access in figures/Schmidt2018_dyn/Snakefile (see #22 (closed))
• fix division by zero errors in analysis_helpers.py (see #21 (closed))
• fix default value of K_stable (see #21 (closed))

@jarsi, could you have a quick look?