Adding ML module and example use

[KMarkert]

Adding machine learning module to geemap based on #124 .

Additional files included:

• geemap/ml.py: ml module which includes functions to convert sklearn ensemble/decisiontree estimator objects to strings for use with ee
• examples/notebooks/local_rf_training.ipynb: example notebook use that highlights functionality
• examples/data/rf_example.csv: csv file containing data used in example notebook.

Updated files:

• requirements.txt: added additional requirements used by the ml model, specifically numpy and pandas

[giswqs]

@KMarkert Thanks a lot for the excellent contribution! Merging now.