Add a more meaningful "ptp"

The peak-to-peak function, ptp(), is a simple and handy function to use, but it confuses the PEP20 suggestions a bit. See this discussion for reference: http://d.pr/i/snXG

Based on the discussion, it may be a good idea to make the name more relevant? Some suggestions include span() or valuerange().

[mattip]

Closing. This is more appropriate for scipy.

[WarrenWeckesser]

FWIW: Over in ufunclab, I created a function called peaktopeak as an exercise in creating generalized ufuncs. peaktopeak makes two imrovements over ptp:

• It makes a single pass over the data, so it will usually be faster than ptp.
• For arrays of signed integers, peaktopeak returns an unsigned integer result. ptp returns a negative value with arrays of signed integers when the peak-to-peak value exceeds the maximum signed integer of the array’s dtype (e.g. np.array([-100, 0, 100], dtype=np.int8).ptp() returns -56).