Overhaul: repo restructuring

[leewujung]

Below is the repo structure @LOCEANlloydizard and I discussed for the overhaul:

|-- src
       |-- echoshader
              |-- elements
                       |-- echogram.py
                       |-- track.py
                       |-- curtain.py
                       |-- geo_grid.py
              |-- app
              |-- core.py   # accessor registration

The idea behind the naming is that we can them use the functions in elements in the app (combination of elements).