21 function to get elevation associated with points

[temospena]

1. Added Support for POINT Geometries (Closes #21)

• Extended elevation_add() to gracefully handle sf objects containing POINT or MULTIPOINT geometries.
• Consistent API: By default (add_z = TRUE), the function embeds the extracted elevation directly into the point's geometry as the Z coordinate (returning POINT Z features), mirroring the behavior for linestrings.
• Flexibility: Added an add_column = TRUE/FALSE parameter for points, allowing users to optionally append the elevation as an explicit data frame attribute column. Setting add_z = FALSE and add_column = TRUE allows returning a standard XY geometry with the elevation only in the column.

2. Reorganized Scripts & Separated Concerns

• Created a new centralized R/elevation.R file to handle all elevation-related operations (elevation_extract, elevation_add, elevation_get, and sf_mid_ext_lonlat).
• Moved all z_* helper functions from R/z-functions.R into R/elevation.R and deleted the orphaned file.
• R/slopes.R is now strictly dedicated to pure slope mathematical estimations.
• R/slope_get.R is now strictly dedicated to raster/matrix utility functions.

3. Test Suite Refactoring & Coverage

• Created a new tests/testthat/test-elevation.R file and migrated all elevation/Z-coordinate tests over from test-slopes.R, test-get.R, and test-z.R (deleting the latter two).
• Added comprehensive unit tests for elevation_add() with point geometries, covering all permutations of the add_z and add_column arguments.

4. Documentation Updates

• Updated the roxygen documentation for elevation_add() to explicitly detail the behavior with points and the new add_column and add_z parameters.
• Added explicit @examples for point geometries directly in the documentation.
• Updated the "Get started" vignette to mention that elevation_add() also works natively with points.