JS formula prop available in Python #58

Description

formula prop available in Python, though JS formulae are unusable from Python.

Steps/Code to Reproduce

In the python REPL run:

help (dash_vtk.Calculator)

We see there is a prop formula available, but JS formulae are unusable from Python

Expected Results

Only have props that can be used in Python

Actual Results

formula is available.

Versions

Dash 2.0.0
:3: UserWarning:
The dash_html_components package is deprecated. Please replace
import dash_html_components as html with from dash import html
import dash_html_components; print("Dash Core Components", dash_html_components.version)
Dash Core Components 2.0.0
:4: UserWarning:
The dash_core_components package is deprecated. Please replace
import dash_core_components as dcc with from dash import dcc
import dash_core_components; print("Dash HTML Components", dash_core_components.version)
Dash HTML Components 2.0.0
Dash VTK 0.0.9

@alexcjohnson