[BIGDL 2.0] finding a way to easily setup development envrironment

[yangw1234]

PYTHONPATH or pip install -e does may not work because bigdl package is spread into multiple sub-projects.

Building and installing each time we have a change is too heavy.

Currently two options are avaiable:

1. merge the source tree, all sub project keep the bigdl package in a central place
2. rename each sub projects' root package to a different name, e.g. "bigdl-nano", "bigdl-orca"