Normalize input and output

[thuiop]

Now that the structure of BTK is mostly fixed, we should address some issues regarding the input and the outputs.

• The output for multiple surveys is like blend_results["blend_images"]["LSST"] while the output for several measure functions is like measure_results["sep_measure0"]["deblended_images"]. I think we should swap the order for the surveys.
• We should think about how to handle the case where only one survey/measure function/measure_kwargs is inputted. Right now, surveys must be given as a list, but does not take the "dictionary form" when there is only one survey. For measure functions, the input is always a list and the output is always a dictionary index by the names of the functions. For the measure_kwargs, the input is a list (or None) and the output adds numbers to the end of the measure function name (hence the "sep_measure0"), even when the input is None or a list of length one. I personally feel that the best would be to have the possibility to give a single element and get the results in the "simple form", or give a list of elements and get the results in the "dictionary form" ; for the measure_kwargs this would mean not adding the zero in the name ; it would add some code though to do all the checks though.
• As pointed in Do we need both meas_bands in DrawBlendGenerator and meas_band_num in MetricsGenerator? #152 , the meas_bands are not really handled correctly right now
• There may be other redundant/useless parameters