Wrapper for grdtrack

[weiji14]

Description of the desired feature

Implement grdtrack which "Sample grids at specified (x,y) locations". Continuing to add data processing functionality to pygmt following my earlier work on #245.

The grdtrack function belongs to the "Sampling of 1-D and 2-D Data" family of functions in GMT (see module quickref). It is slightly tricky to implement as it takes 2 input files (a table and a grid), but is nevertheless useful for people working on stuff like radar or laser altimetry, as mentioned by @steo85it in https://gitter.im/GenericMappingTools/pygmt?at=5c89603a8f294b134a38ed82.

Notes

• May want to future proof the code to share it with other sampling functions like sample1d, grdsample, etc.
• Test cases will require new sample datasets. hawaii_topo.nc would be nice but I can't find it, so I'm suggesting @ridge.txt and @spac_33.nc used in https://gmt.soest.hawaii.edu/doc/latest/gallery/ex33.html.
• Handle I/O better, as mentioned here. Should we separate data reading/writing from data processing? I.e. have pygmt function running only on pandas tables, xarray grids.

Idea for structure

Store the grdtrack function under sampling.py. Similar to how e.g. surface is under gridding.py.

# File at gmt/sampling.py
"""
GMT modules for Sampling of 1-D and 2-D Data
"""

def grdtrack(table: pd.DataFrame, grid: xr.DataArray, **kwargs):

Are you willing to help implement and maintain this feature? Yes