Manage exported functions

Currently, all the functions are exported to facilitate development. Thus, they are listed in the manual.
However, it is not relevant to do so, especially for the plot*() functions.

In addition, I am not sure that it is relevant to create plot S3 methods for HN, RR and TA objects just to manage the type of plot.
Maybe it could be managed inside the plot.ts_meteo() function. It could simplify the readability of the index of the package functions for the user. Especially since the user never consciously manipulates these objects.
Maybe it is enough to keep only plot.ts_hydro() and plot.ts_meteo().
Maybe it could be the same for the regime S3 methods.

I don't know if we should split the 'hydro' and 'meteo' functions.
I think that it more readable and easier to manage, but the code and documentation are sometimes a bit redundant.
The 'meta()function is not currently splitedmetats_hydro()andmeta.ts_meteo()`.
By the way, it would be possible to detect automatically if it is a hydrological or meteorological feature by checking the feature code:

# hydro
grepl("^\\d", "Y1234567")
[1] FALSE

# meteo
grepl("^\\d", "12345678")
[1] TRUE