implement decorator for deprecating function parameters

Also implement a specialization of this decorator for the 'args'
parameter, so as to avoid boilerplate (i.e. redefining the help
string everywhere the decorator is used).

The deprecation decorator can also be called directly with no
parameters to raise the warning. This is necessary because
methods of Cython cdef classes do not seem to play well with
this decorator (even after applying @cython.bind(True)), so
we need to call it directly from the body of the method.