Bug/b value estimation

[aronsho]

I worked on two issues here:

1. The a/b-value estimators gave an incorrect error
   -> when there was no magnitudes above mc, it gave back a binning error, this is fixed now.
   -> While I was on it, I also included an error if there were nan values
   -> Also, I changed the order: first sanity check, then filtering

2. I included the possibility of including a time vector for the b-positive methods
   -> Now, the magnitudes do not have to be given sorted if the time is given
   -> While on it, I included the capability of including the index as an attribute of the class. Now, it is very easy to retrieve the actual magnitudes used: magnitudes[estimator.idx]. This is super useful also, when there is some other array that we want to filter in the same way as the magnitude differences are.

@lmizrahi : Would be cool if you used the estimator and checked if it works correctly
@schmidni : I messed a bit with the estimator classes, so I thought it would be good to have you loook at it :)

[schmidni]

some coments I just made in one place, but should also be applied in other places (like when you did the same changes in a and bvalue estimators, or error types and so on.

[aronsho]

Ok, I believe I addressed all the points, hope that we can merge this one, too! :)

[schmidni]

Thanks for your patience :) great work!