feat/activations: add in-place activations

Activations can now be calculated in-place, requiring less memory. To use it, the same blob name should be supplied as input and output to a activation layer.

Example:
```rust
// set up linear1 layer
let linear1_cfg = LinearConfig { output_size: 1568 };
let mut lnr1_cfg = LayerConfig::new("linear1", LayerType::Linear(linear1_cfg));
lnr1_cfg.add_input("data");
lnr1_cfg.add_output("linear1_out");
net_cfg.add_layer(lnr1_cfg);
// set up sigmoid layer
let mut sigmoid_cfg = LayerConfig::new("sigmoid", LayerType::Sigmoid);
sigmoid_cfg.add_input("linear1_out"); // same input and output
sigmoid_cfg.add_output("linear1_out"); // same input and output
net_cfg.add_layer(sigmoid_cfg);
```