To-Do list associated with streamlining code and improving quality

[MoAly98]

The following is a to-do list of work that need be done for the pre-processing and histogramming codes to interface nicely and provide a consistent UX:

For sklimming:

• Sklimming should replace the Branch objects with Variable or Observable.
• The idea of Functor sub-class called Builder for a Variable is much neater than the new_branches dictionary.
• Remove the arg_types argument for Variable/Branch objects and replace with an optional list_str_arg where the default assumption when a str is provided in argument becomes that it is a another Branch
• Sklimming back-end is too messy -- especially the organisation of different type of branches (new, tmp, on, off)
• Dask distribution for the reading in the backend in a similar way to the histogramming code.
• Consistent way of outputting yields using a given branch
• Config to use a schema in a similar way to histogramming
• reader code should be a processor code
• Argparser to select branches and samples in stirring script
• Look into a coffea backend
• Make variables/branches optional in Sample constructor -- in case it is needed at only histogramming
• Remove or actually use job name in general settings
• indirs in general_settings to become used as default for samples if no where is given
• Branch/Variable awate of tree rather than using a dict? this way user can just specify which trees to use when
  looking for this variable withour repition
• Better multiple-tree support

For histogramming:

• Provide an easy way for user to change sample settings -- does user really need anythig other than to set regex (i.e tag and specify if a sample isdata)?
• The input_paths finder in InputManager should be able to handle user provided methods of finding paths, possibly via a decorator in the config? This is for users who have NTuples that they want to Histogram without running through pre-prcoessing
• Data rendering before passing awkward arrays to boost_histograms. This probably means handling masked awkward arrays, since it seems like boost_histograms do not deal well with None in the awkward/numpy arrays (they get dumped in the underflow) boost-histogram issue
• Regions should not be required -- only Observables should be (inclusive sample can be selected by a dummy function)
• Overall Systematics
• Observable.fromFunc() does it really need args -- can we not just replace args with var
• access to samples if specified sklimconfig in settings, or no?
• Auto binning (uniform betwen min and max if user not provided binning -- safeguard for user? warn them? flag to tell us its acceptable not to have binning?)
• Do we need general.from_hists flag to allow retrieving histograms from Histogram files?
• [] Histo name goes to :: or __ between different XP components
  For both sklimming and histogramming:
• Remote distribution of jobs with dask(e.g. HTCondotCluster)
• Can Functor fromStr as it is parse slicing syntax?
• Test functions for all features
• Variable and Observable can maybe inherit from a parent class -- what is the benefit a user will get if they have to specify the binning for var by var? except if we support no binning and just do a uniform binning on behalf of user, then yhey can just import their variables from sklimming to histogramming.

[MoAly98]

Mentioning the relevant pull request above which these changes are needed: #35