Start to work on doppler flow. Copy from dca flow.
Add short draft description of Doppler process.
Change doppler-flow.  Add some descriptions. Remove unnecessary comments.
Add doppler-flow and doppler-flow-example.
Merge remote-tracking branch 'upstream/gh-pages' into doppler-flow
Fix some issues reported by @andresmoschini.
Merge pull request #40 from easla/doppler-flow

Add doppler-flow and doppler-flow-tutorial.