Spectrum-plotter

Added ability to convert units of X and Y axis

**Full Changelog**: https://github.com/fusion-energy/spectrum_plotter/compare/v0.1.0...v0.1.1

openmc tally objects can be passed in as the input spectra.

Adds the option to plot the graph with Plotly (in addition to Matplotlib). Plotly offers a drop down menu to change the axis scales.

fixed bug that removed leading zeros from y data
refactored to share common code between plot_spectrum and plot_spectra

Initial release with minimal viable product plotting spectra / spectrum