Make adding summary data optional in `draw_ldc`

[mps9506]

Currently draw_ldc requires the summary data argument point to the output of summ_ldc in order to add the median measured loading points to the graph. This should be optional for two reasons. (1) Users have the option to draw the ldc easily without additional summary data. (2) If there is a desire to overlay output from other daily loading estimate methods (LOADEST, WRTDS, GAMs etc.), that can be manually added to the output of draw_ldc (I think).

To do:

• make the .tbl_summ argument optional
• add examples of incorporating GAM or LOADEST daily load estimates to the output of draw_ldc (this probably should be a pkgdown article)