refactor z-calculation code

[behinger]

`

			function calculate_z(::MultiModelAmica,z,v)
				# xxx
			end

			calculate_z(::MultiModelAmica{<:MixedGeneralizedGaussian},z,v) = v[h,:] .* z # 	sumz = sum(myAmica.z[i,:,j,h])
			
				
			calculate_z(::MultiModelAmica{<:GeneralizedGaussian},z,v) = v[h,:] # sumz = sum(myAmica.z[i,:,j,h])

			calculate_z(::SingleModelAmica{<:MixedGeneralizedGaussian},z,v) = z # 	sumz = sum(myAmica.z[i,:,j,h])
			calculate_z(::SingleModelAmica{<:GeneralizedGaussian},z,v) = z # sumz = sum(myAmica.z[i,:,j,h])

			normalize_z(::Union{MultiModelAmica,SingleModelAmica{<:MixedGeneralizedGaussian}},z,sumz) = z ./ sumz
				
			
			


			z = calculate_z(myAmica,myAmica.z[i,:,j,h],v)
			sumz = calculate_sumz(z)

			sumz <= 0 ? continue : ""
			
			myAmica.z .= normalize_z(myAmica,z,sumz)