Implement Fisher matrix determination of covariance matrix

I have implemented a new flag for Montepython to generate a covariance matrix using an inverse of the fisher matrix estimate. This is supposed to be fast, than having to run small chains to determine a better covariance matrix.

The new argument is:
--fisher 
0: default
1 : Enables the Fisher matrix determination

I have tested the code by running it on the BAO dataset, it does properly walk the degeneracy direction even when using simple Metropolis-Hastings algorithm. 

More extensive tests may be needed. Also documentation needs to be improved.

Surhud