Dynamic Time Warping, organise into library + user code

[Roly Perera]

Minor cleanup of Dynamic Type Warping example:

• lib/dtw module (analogous to lib/convolution)
• lib/fnum module for FNum support functions
• Drop f prefix from FNum support functions (where possible)
• Parameterise dtw on cost function
• User code (invoking library code and providing cost function and input sequences) in example/slicing/dtw
• Remove any remaining files from example/dtw
• Other misc. cleanup
• If array indexing would be better zero-based (for consistency with nth), create a new issue

See also:

• #781