v0.8: More configurable parameters for generating counterfactuals and better input checking

• Better error checking for user input parameters
• Improved error messages for feature importance functions
• Bug fixes for dimension mismatch with categorical variables
• Add capability to serialize and de-serialize dice-ml explainers
• Add more configurable parameters to generate_counterfactuals function
• Increased test coverage
• Replaced boston housing with california housing dataset
• Tests and support for Python 3.10, removed test support for 3.6

Contributors: @gaugup, @amit-sharma