create_uptake_matrix: Create the uptake matrix, the right-hand part of equation 2
In richelbilderbeek/huisweis: Reproduce Huisman And Weissing 1999

Create the uptake matrix, the right-hand part of equation 2

1 2	create_uptake_matrix(srcs, species_densities, resource_densities, max_growth_rates, hsrs, mortality_rates)

`srcs`	a matrix of species' resource content of species (rows) of resources (columns). Can be created by create_srcs
`species_densities`	density of all species
`resource_densities`	resource densities
`max_growth_rates`	max growth rate of all species
`hsrs`	a matrix of half-saturation constants of species (rows) on resources (columns). Can be created by create_hsrs
`mortality_rates`	mortality rates of all species