Add Kalman Smoothing derivatives

[Jacob-Stevens-Haas]

I added a kalman smoothing global derivative method. Example:

import numpy as np
from matplotlib import pyplot as plt
from derivative import dxdt
t = np.arange(0,2*np.pi, .05)
x = np.sin(t) + .1 * np.random.randn(*t.shape)
xdot_hat = dxdt(x, t, kind="kalman", alpha=1)
print(np.max(xdot_hat-np.cos(t)))
plt.plot(t, np.cos(t), "b-", label="true derivatives")
plt.plot(t, xdot_hat, "bx", label="smoothed derivatives")
plt.plot(t,x, 'gx', label="noisy measurements")
plt.plot(t, np.sin(t), 'g-', label='true values')
plt.legend()

Passes all tests, but I can't build the docs. I get:

Running Sphinx v3.5.4
WARNING: html_static_path entry '_static' does not exist
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 5 source files that are out of date
updating environment: [new config] 5 added, 0 changed, 0 removed
reading sources... [100%] notebooks/Examples                                                         
Notebook error:
PandocMissing in notebooks/Examples.ipynb:
Pandoc wasn't found.
Please check that pandoc is installed:
https://pandoc.org/installing.html
make: *** [Makefile:20: html] Error 2

Is pandoc missing from pyproject.toml? Working on that now.