Added support for NChannelGeneratorSheet and optimized color conversion


Optimized colorconversion added to topo.optimized.color (new file) and topo.analysis.__init (overriding featuremapper.colorspaces default unoptimized functions).

NChannelGeneratorSheet added to topo.base.generatorsheet.

This code is a  **really** terrible idea - replacing a simple numpy operation with something non-standard. Just import numpy here as follows:

```python
import numpy as np
```

I've listed what to replace ``clip_upper`` with in a comment below.