add metadata for ETOPO depth

[bselden]

Add this metadata for ETOPO depth (object "depth") in trawlData package

From https://www.ngdc.noaa.gov/mgg/global/relief/ETOPO1/docs/ETOPO1.pdf

Table 1: Specifications for ETOPO1.
Versions: Ice Surface, Bedrock
Coverage Area Global: -180º to 180º; -90º to 90º
Coordinate System: Geographic decimal degrees
Horizontal Datum: World Geodetic System of 1984 (WGS 84)
Vertical Datum: Sea Level
Vertical Units: Meters
Cell Size: 1 arc-minute
Grid Format: Multiple: netCDF, g98, binary float, tiff, xyz

[rBatt]

Good call Becca.

Some of that table is only relevant to the raw data, though. I'll focus on documenting the R object.

Also, it seems that the data are in 2 minute resolution, not 1 minute.

#' @details
#' \tabular{ll}{
#' Cell Size \tab 2 minute (official doc says 1 arc-minute, but the data are actually 2 minute) \cr
#' Vertical Datum: \tab Sea Level \cr
#' Vertical Units: \tab Meters \cr
#' Longitudinal Range: \tab -190 to -40 ('western hemisphere' units) \cr
#' Latitudinal Range: \tab 20 to 65 \cr
#' }
#' 
#' @source \href{http://www.ngdc.noaa.gov/mgg/global/relief/ETOPO2/ETOPO2v2-2006/ETOPO2v2c/netCDF/}{ETOPO Depth Data Acquired from NOAA}
#' 
#' @references
#' \href{https://www.ngdc.noaa.gov/mgg/global/relief/ETOPO1/docs/ETOPO1.pdf}{ETOPO Documentation}