Config file usability improvement -- environment variables

[ceholden]

Allow environment variables to be used in configuration files. Should greatly help usability in situations where you want to run the same configuration on different but identically formatted datasets. Previously this was accomplished by sed'ing many file paths in the config files.

Example usage, where $ROOTDIR is used in the configuration files to point to the dataset location:

export ROOTDIR=$HOME/Documents/landsat_stack/p013r030/images 
yatsm -v pixel $CONFIG/envvar.yaml 25 25

[ceholden]

Added in 0683e03 and (partially) documented in 35a147d