types `activation_strategy_t`, `swish_t`, `relu_t`, `sigmoid_t` in
order to keep the elemental keyword.

`differentiable_activation_strategy` and
`trainable_engine%to_inference_engine` because they contain
logic that is not allowed in pure procedures.

when compiling with nagfor. These directives should be removed
when, as expected, nagfor provides support for `random_init` in
an upcoming version.