Code cleanup

    Rewrite function GetLeafNames
    Use solution proposed by @dennisklein in #1419

    Fix variable names.
    Remove C-style casts.
    Use uniqe_ptr where possible.
    Remove fixed tree name.
    Use standard types instaed of ROOT types.
    Use references instead of pointers where possible.
    Remove unneeded functions.
    General code improvements.

Author: fuhlig1