Add 'csaps' shortcut function

[espdev]

This function might be used in procedural code as useful shortcut for all supported data smoothing cases.

The example:

import numpy as np
from csaps import csaps

x = np.linspace(-5., 5., 25)
y = np.exp(-(x/2.5)**2) + (np.random.rand(25) - 0.2) * 0.3
xi = np.linspace(-5., 5., 150)

# Smooth data with smoothing factor 0.85, return smoothed values
yi = csaps(x, y, xi, smooth=0.85)

# Smooth data and compute smoothing factor automatically, return smoothed values and computed smoothing factor
yi, smooth = csaps(x, y, xi)

# Do not evaluate the spline, only compute it, return smoothing spline representation object
sp = csaps(x, y, smooth=0.98)