add a function for the derivative of ceil and floor

[burcin]

We should define a new symbolic function for the derivative of ceil or floor.

In Maple:

> diff(floor(x),x);
                                  floor(1, x)
> diff(floor(x),x,x);
                                  floor(1, x)
> diff(ceil(x),x,x); 
                                  floor(1, x)
> eval(diff(ceil(x),x,x),x=1.5);
                                       0
> eval(diff(ceil(x),x,x),x=0);  
Error, (in floor) floor is not differentiable at integers

[ktkohl]

These give the right answer but also a runtime error:

sage: floor(x).derivative().subs(x=1.5)
Exception RuntimeError: 'maximum recursion depth exceeded while calling a Python object' in 'sage.structure.parent.good_as_coerce_domain' ignored
0
sage: ceil(x).derivative().subs(x=1.5)
Exception RuntimeError: 'maximum recursion depth exceeded while calling a Python object' in 'sage.structure.parent.good_as_coerce_domain' ignored
0

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New commits:

​8013b9f

Added ceiling and floor functions to allow 2nd argument for derivative.

[rws]

Just a quick note that there are empty examples and tests fields but you probably know that.