Use direct body density calculation, rather than inverse function

[ryanjdillon]

The current calculation of body density uses a function translated from Martin's code which calculated the density solely from the % fat/lipid, and though the same, I'd like to do the full calculation by also passing the % body water.

In smartmove/ann/prey.py/add_rhomod():

    dens_gcm3 = pyotelem.physio_seal.lip2dens(isotope['fat_perc'])
    isotope['density_kgm3'] = dens_gcm3 * 1000