Remove ray from required dependencies of lightsaber

[pchakraborty1]

required version of ray is not on conda-forge which creates a problem for the conda-forge recipe of dpm360-lightsaber.

This will also reflect the fact that lightsaber can work without ray installed

Todo:

• remove ray from requirements.txt and move it to a tune section in lightsaber::setup.py
• add ray checking like pysurvival