fixed writing hist file problem for Reconstructed class

Updated plot_models.py and used for testing.
Merge remote-tracking branch 'origin/master'
created a script to produce SNO+ spectral plot (spectral_plot.py)

Currently, not quite correct - need to modify production.py so that i can make
FV cut. Also includes some fairly major refactoring in the way default values
are handled. Created a new module to hold defualt values for parameters.
Created a new class RoIUtils to handle ROI manipulations.
refactored code so that multiple histograms can now be stored im memory

Changed SpectrumData._histogram --> SpectrumData._histograms, a dictionary
containing all the histograms in memory (as values), with the corresponding
keys being the (ROOT) label of the histogram.