Data Loading Scripts Improvements for bash variables

[denisekgosnell]

This ticket requests two improvements:

1. implement a more robust way to set the DSBULK_PATH variable
2. rename DEST_KS into DEST_GRAPH
3. update the READMEs to reflect the changes to usage

All scripts are written such way, so it's just enough to set DSBULK_PATH outside of the scripts and it will be picked up, for example, as DSBULK_PATH=.... data/ch4/load_ch4.sh.

Or just use export DSBULK_PATH=.... to set it. Another possibility is to add dsbulk-1.4...../bin to the PATH, like export PATH=path_to_dsbulk_bin:$PATH.

All of this is easier than to modify all scripts, like the current implementation requires.

Similar approach could be used for DEST_KS for every script.