Control variables in land DA workflow scripts with an external file

[zhichang-guo]

The current workflow scripts contain a lot of variable declarations which differ among users and experiments. This makes PR merging process complicated, users have to make a lot of changes to the workflow scripts when they do certain experiments, and they have to have one set of modified workflow scripts for each specific experiment. I implemented a method to control the bash variables with an external file and separate the variable declarations from the workflow scripts. With the changes it is much easier for the users to share the workflow scripts and data, to separate the experiments from the source codes and scripts, and to use the same set of workflow scripts for a lot of different experiments.