Volatility Surface

The user inputs:

2 scalars:
- an annualized risk-free rate
- the current price of an underlying asset

3 vectors:
- a vector of time to maturity
- a vector of strike prices
- a vector European call prices gotten from the market for the same underlying asset.

The function VolSurface.m will then:

- compute and output the Black-Scholes implied volatility (this will be a matrix).
- get and plot the corresponding volatility surface using a kernel (Gaussian) density estimation.