Use the directory structure to more obviously expose the user API

[rouson]

@milancurcic I keep feeling silly when I realize things that probably should have been obvious. It was very helpful that your comment on PR #71 pointed me to the nf module for the user API. I recommend using a directory structure first suggested to me by @everythingfunctional and which I've subsequently adopted on all libraries that I develop. For neural-fortran, the directory tree would look like the one below. I'll submit a new PR that organizes

% tree src 
src
├── nf
│   ├── nf_activation.f90
│   ├── nf_base_layer.f90
│   ├── nf_base_layer_submodule.f90
│   ├── nf_conv2d_layer.f90
│   ├── nf_conv2d_layer_submodule.f90
│   ├── nf_datasets_mnist.f90
│   ├── nf_datasets_mnist_submodule.f90
│   ├── nf_dense_layer.f90
│   ├── nf_dense_layer_submodule.f90
│   ├── nf_input1d_layer.f90
│   ├── nf_input1d_layer_submodule.f90
│   ├── nf_input3d_layer.f90
│   ├── nf_input3d_layer_submodule.f90
│   ├── nf_io.f90
│   ├── nf_io_submodule.f90
│   ├── nf_layer.f90
│   ├── nf_layer_constructors.f90
│   ├── nf_layer_constructors_submodule.f90
│   ├── nf_layer_submodule.f90
│   ├── nf_loss.f90
│   ├── nf_loss_submodule.f90
│   ├── nf_maxpool2d_layer.f90
│   ├── nf_maxpool2d_layer_submodule.f90
│   ├── nf_network.f90
│   ├── nf_network_submodule.f90
│   ├── nf_optimizers.f90
│   ├── nf_parallel.f90
│   ├── nf_parallel_submodule.f90
│   ├── nf_random.f90
│   └── nf_random_submodule.f90
└── nf.f90

[milancurcic]

Thanks for the suggestion. I'm not in favor of this change at this time. This is putting almost all source files one level down, only to indicate which module should be used in applications. If it's not clear enough from the examples then let's improve the (missing) user docs.

[milancurcic]

I changed my mind about this after our call. It's one way to help make the user API clearer to newcomers, so let's do it.