Fast projections

Added FastProjectX and FastProjectY methods to Plotter2D. These copy
the contents from a 2D histogram into the 1D projection histogram.
Because the root tree is never re-scanned, this method is much faster
than the normal ProjectX/Y methods.

The SaveHist methods in Plotter1D/2D will now append the histogram
to a file that already exists instead of overwriting the file.

Added overloads to SaveHist which allow the saving of an external
histogram to a file.

Removed the 'filled' flag since it was no longer needed.

Plotter will output a message that prompts the user to fill the 2D
histogram before making projections.

Plotter can now fill 1D histograms using either the X or Y axis
settings. The number of bins and the range of the 1D histogram is
automatically set before filling.

Removed all branch options from Plotter since Plotter1D/2D no longer
support filling of histograms from branches of the main tree.