Unspecified variable M_China in transmission.jl causes error in test

[TheHelenofTroy]

M_China is an unspecified and undeclared variable in https://github.com/SamuelBrand1/KenyaCoV/blob/master/src/transmissionmodel.jl#L175

This will cause function calling of calculate_R₀_scale to fail, even when M_China is specified in the global scope. See https://github.com/SamuelBrand1/KenyaCoV/blob/master/tests/run_KenyaCoV_agestructuredmodel_vs_2.jl#L18 where M_China is named, but https://github.com/SamuelBrand1/KenyaCoV/blob/master/tests/run_KenyaCoV_agestructuredmodel_vs_2.jl#L18 and https://github.com/SamuelBrand1/KenyaCoV/blob/master/tests/run_KenyaCoV_agestructuredmodel_vs_2.jl#L31 will still error out with UndefVarError.