set start month in knn_space_time_disagg()

[rabutler-usbr]

for the rownames of the output, we currently assume that the data are always starting in January.

need a way to specify this.

could:
1 - require that the mon_flow have rownames or a columns with some monthly info
2 - add another argument that just states the start month

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plotting

• add start_month to knnst objects
• nothing changes on monthly stats, just need to update get_pattern_flow_data_df() so that it uses start_month and does not assume it starts in January
• new function to compute "agg_year" based on start_month. Idea being that we assume that if data start in a certain month, that is the beginning of the "agg_year", e.g., water year or fiscal year.
• update get_ann_plot_stats() to sum by agg_year and not by year.
• update plotting order of monthly boxplots to also be based on start_month
• new test - plot data with 2 different start months (use n = 1), get the annual data out of the ggplot object, it should be the same

[rabutler]

will need to update the plotting of historical data in plot.knnst()

may also want to add in option to change the order the months are shown. Right now it's always Jan - Dec

[rabutler-usbr]

Decided to go with option 2 - added start_month argument to knn_space_time_disagg(). It seemed a bit complicated to require a certain format of row names at this point.