add_water_depth: Add water depth
In dswdejonge/TripleD: Construct a TripleD database.

This function adds an attribute with water depth to the stations dataframe extracted for a certain position (lat, lon) based on batymetry.

1	add_water_depth(stations, bathymetry, col_lon, col_lat, col_name)

`stations`	Dataframe (tibble) with the attributes "Lat_DD_midpt" and "Lon_DD_midpt".
`bathymetry`	An xyz dataframe (tibble) or object of class bathy created by the marmap package.
`col_lon`	Column name with longitudes in stations dataframe
`col_lat`	Column name with latitudes in stations dataframe
`col_name`	New column name in which the extracted depth will be added.