Make `matchGrowth()` work when `n != p`

[gustavdelius]

Currently matchGrowth() simply rescales the search volume, the maximum consumption rate and the metabolic rate
all by the same factor in order to achieve a growth rate that allows individuals to reach their maturity size by their maturity age while keeping the feeding level and the critical feeding level unchanged. Unfortunately this works only when the exponent n of the intake rate is the same as the exponent p of the metabolic rate.