Wave spectra

[SChristineP]

New plots/spectra.py with following functions:

• plot_spectra_1d : Plots 1D spectra aggregated by month, for a specific month across multiple years, or for a single timestamp.
• plot_spectrum_2d : Plots a 2D directional spectrum for a specific month, time period or single timestamp.
• plot_diana_spectrum : Generates a Diana plot showing the 2D wave spectrum.
• plot_spectra_2d : Creates a 12-panel plot of aggregated 2D spectra based on method.

New tables/spectra.py
Added functions in stats/spec_funcs.py
Removed plot_spectrum_2d function in dir.py (now covered by plot_spectrum_2d in plots/spectra.py)
Added test functions for the plotting functions.