Refactor climdex.r into modular R scripts

[QSparks]

Summary

This PR refactors the original climdex.R file by breaking it into multiple smaller, focused R scripts.
There are no changes to the functionality; this is purely an organizational refactor.
R CMD check runs without errors, warnings, or notes and all tests pass.

Changes Made

1. Refactoring into Modular Scripts:

   • The code has been divided into several scripts based on functionality and were organized according to this comment:
     • climdexInput_class.R: Contains the definition of the climdexInput class and its associated methods, and validation routines.
     • precipitation_indices.R
     • temperature_indices.R
     • threshold_indices.R
     • date_utils.R
     • series_utils.R
     • stats_utils.R
     • threshold_utils.R

2. Documentation Updates

   • DESCRIPTION File:

     • Added Encoding: UTF-8 to handle ✖ roxygen2 requires "Encoding: UTF-8" error
     • Addstats and utils to imports to handle errors such as: no visible global function definition for ‘read.csv’
     • Added Roxygen: list(markdown = TRUE) and updated RoxygenNote to version 7.3.2.

   • climdex.pcic-package.R:

     • Add .registration = TRUE to the @useDynLib tag so that roxygen2 would add it the namespace.
     • Updated deprecated @docType "package" to _PACKAGE.
     • Ensured that roxygen2 Generates Correct Namespace:
     • Created with roxygen2::roxygenize(clean=T)

     resolves: Refactor Climdex.r into Submodules #35