* ML model improvement : Adding "SHOW MODELS and DESCRIBE MODEL"

Author:    rajagurunath <[email protected]>
Date:      Mon May 24 02:37:40 2021 +0530

* fix typo

* ML model improvement : added EXPORT MODEL

* ML model improvement : refactoring for PR

* ML model improvement : Adding stmts in notebook

* ML model improvement : Adding stmts in notebook

* ML model improvement : also test the non-happy path

* ML model improvement : Added mlflow and <With> in sql for extra params

* ML model improvement : Added mlflow and <With> in sql for extra params

* Added Test cases for Export MODEL

* Added ML documentation about the following:
1. SHOW MODELS
2. DESCRIBE MODEL
3. EXPORT MODEL

* refactored based on PR

* Added support only for sklearn compatible models

* excluded mlflow part from code coverage

* install mlflow in test cluster

* Added test for non sklearn compatible model

* Added: initial draft of referencing multiple schemas

* Added schema DDLs
1. Create Schema
2. Use schema
3. Drop schema
4. Added testcases

* Use compound identifier for models, tables, experiments, views

* Split the compound identifiers - without using the schema so far

* Added a schema_name parameter to most functions and actually use the schema

* Pass on the schemas to JAVA

* Some simplifications and tests

* Some cleanup, documentation and more tests (and fixed a bug in
aggregation)

* Remove unneeded import

Co-authored-by: gurunath <[email protected]>
Co-authored-by: Gurunath LankupalliVenugopal <[email protected]>