batch changes to saxs_merge

- remove --lambdamin option
- set starting value of A to mean intensity in model 'Flat'
- after fitting, validate fit by checking if variances are positive
- make fitting more robust by always trying multiple values for sigma,lambda and
  tau
- raise an error if all bayes factors are inf
- add write_fit function for debug write of profile+fit
- fix issue wrt bonferroni and verbose