Renamed 'using' to 'applying' - clearer than both previous names

The name of this utility should now be settled. In common useage
'applying' and 'using' have similar semantics but 'applying' has the
more correct technical term (see Python's apply built-in).
Reworked 'applying' to behave more like an idiomatic decorator

The 'applying' utility now returns an instance of itself (wrapping the
given function) when used with decorator syntax. Previously, a
callable object was not returned, defying idiomatic decorator style.
Numpy no longer a core dependency

Lancet now works with with numpy types when available without
depending on numpy. Now using a pure Python replacement for
np.linspace.
Decorator classes now need to be explicitly called

The 'applying' and 'review_and_launch' decorators no longer invoke the
wrapped callable upon decoration. Instead they return self as a
callable to be explicitly invoked (more idiomatic). Latter decorator
now has appropriate str, repr and pretty-printing methods.