Move the long term seasonal to the data.py module

[UchechukwuUdenze]

• Move it out of the plot function and have it exist on its own
• it will probably be its own function
• include optional arguments to also add it to the generate_dataframes functions as one of its 'aggregates'
• Clean up the plot_seasonal function (maybe remove it completely and just use plot)

[UchechukwuUdenze]

When dealing with the long term seasonal you might want to have multiple dataframes for mean, min, max, median