Some functions and changes

[vincenzocoia]

This pull request comes with three changes:

1. A modified version of ch_regime_plot() (ch_regime_plot2()) so as not to depend on a specific data frame format as an input.
   • For example, can't easily use the original function if data are retrieved from tidyhydat.
2. A function I like using called ch_ams_timeseries() to visualize a time series of annual maxima. Especially useful for making implicit NA's explicit, so that the line plot breaks when data for a year is missing.
3. A scale_*_gumbel*() family of functions meant to be added to a ggplot object to put return period or AEP on reduced Gumbel variate spacing.

Regarding the regime plot, this is meant to generate a discussion regarding what to do about this new way of engaging with the function, especially since other plotting functions have the same problem.