Covariance in nestlc

Added changes to fitting.py:

- Added 'flatten_result' to _all_
- Changed the estimate of std (errors) in nest_lc() so that the estimate is unbiased
- Added res['cov'] which will hold the covariance of the varied parameters. 

Tests: 
- Compared standard deviation^2 to diagonal elements of the covariance matrix (exact match)
- Compared standard deviation after to standard deviation before (small changes, because these are different runs of nest_lc()? )
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