Create CLI running xclim indicator on spatial subset

[huard]

Create a command line interface for a few indicators

Mandatory inputs

• List of paths to netCDF files

Optional inputs

• Spatial and temporal subset instructions
• Indicator parameter values
• Mapping between expected variables and variable names in the netCDF files

Output

• Path to computed indicator (netCDF)

Cases

• Multiple variable inputs in the same file
• Multiple variable inputs in different files
• Multiple output variables