Improvements to scaling model and plotting

[gustavdelius]

• Included latest version of add_species() from adsp branch
• Added a 'power' argument to plotSpectra to plot spectra involving different powers of weight
• Added parameter checks to set_scaling_model()
• Now using analytic considerations to set normalisation of species abundances
• The initial conditions are now closer to steady state and that is reflected in a tighter bound in the unit test for set_scaling_model()
• Small fixes and improvements to documentation