calcSigma: Calculate variance-covariance matrix
In MJAlexander/distortr: Temporal smoothing methods for demographic time series

Calculate variance-covariance matrix between two vectors, based on squared exponential kernal.

1 2	calcSigma(X1, X2, cov.method, range = 10, smoothness = 2, l = 1, tau = 1)

`X1`	first vector of x values
`X2`	second vector of x values
`cov.method`	either `sqexp` (squared exponential) or `matern`
`range`	parameter for Matern function. Default is 10.
`smoothness`	parameter for Matern function. Default is 2.
`l`	the length scale parameter. Determines the 'wigglyness' of the fluctuations for squared exponential.
`tau`	the amplitude parameter. Determines the magnitude of fluctuations over time.