Option to allow passing user input sacc file for likelihood object etc

[am610]

1. User can now pass their own sacc file as argument, for generating SN likelihood object.
2. Updated features in sacc generattion code. A sub variant of the TD hubble diagram file structure is readable now. For user input HD and covariance matrix, output sacc file naming convention updated : it will now follow the name prefix of the input covariance matrix.
3. Updated Readme file.