docs(init): pass Human-GEM's spontaneous reactions to prep_init_model

[edkerk]

• The Python example for preparing Human-GEM reads the spontaneous column of reactions.tsv and passes it to prep_init_model, as the MATLAB example does.
• A note explains why: ftINIT keeps spontaneous reactions in every extracted model. In Human-GEM these include diffusion transports (cytosol–nucleus, and gases and water across membranes; feat: flag diffusion transports as spontaneous so ftINIT keeps them SysBioChalmers/Human-GEM#1101). Without them, ftINIT can drop such transports in some samples and create artefacts such as DCK becoming essential.
• prepHumanModelForftINIT is described as needing Human-GEM 2.0.1 or later.