Documentation issue : RTNCovariance constructor initializes the covariance matrix with NaN.

I noticed during tests on the CDM parser that the parsed RTN covariance matrix contained NaN for absent values in my test CDM so i went to check if it was normal and found out that this comes from the RTNCovariance constructor.

However, the documentation doesn't specify this so i thought it could be interesting to mention it. In addition, only the lower triangle part of the matrix is filled with NaN, the rest is filled with zeros.