Allow users to specify n_plankton=0

[nanophyto]

This is not very relevant for the NPZD example, but once we implement more plankton functional types, it would be desirable if the user can specify some n_plankton to be 0.

It seems that currently, setting a plankton to 0 breaks things:

    @testset "n_zoo passed as 0" begin

        N2P0ZD = construct_size_structured_NPZD(n_zoo=0)
        model = N2P0ZD()

        @test !iszero(model(Val(:N), 0, 0, 0, 0, P1, P2, N, D, PAR))
        @test !iszero(model(Val(:D), 0, 0, 0, 0, P1, P2, N, D, PAR))
        @test !iszero(model(Val(:P1), 0, 0, 0, 0, P1, P2, N, D, PAR))
        @test !iszero(model(Val(:P2), 0, 0, 0, 0, P1, P2, N, D, PAR))

    end

[radka-j]

Good point! This should just be a matter of including an if statement in the relevant constructor somewhere that skips those plankton groups when the n=0.