Soilgrids

- add 'wrb' map service
- support to show variable units info for the map service

- update documentation for code example (plot boundary) and parameter info (local_file, width, height)
- support to load a local tif file that was originally downloaded from SoilGrids system
- replace xarray with rioxarray for dealing with tif file
- bmi get_value_at_prt() supports add_offset and scale_factor
- update cli with new parameter

- update config.yaml file format
- update bmi time related methods for steady state data
- update update() method for steady state data

This is the first release of the soilgrids package.