Expose maxpool2d constructor in nf

Author: milancurcic

src/nf.f90

@@ -1,6 +1,6 @@
 module nf
   use nf_datasets_mnist, only: label_digits, load_mnist
   use nf_layer, only: layer
-  use nf_layer_constructors, only: conv2d, dense, input
+  use nf_layer_constructors, only: conv2d, dense, input, maxpool2d
   use nf_network, only: network
 end module nf