Diagnostics refactor

[jerrymhuang]

Refactored all diagnostics modules:

1. Created helper functions for common procedures shared by SBC-related diagnostics modules;
2. Created a more flexible plot_distribution_2d() module for pairplots;
3. Migrated all diagnostics modules and related utilities to experimental;
4. Made all diagnostics modules more dependent on Keras;
5. Add data types for all module inputs (except samples; in discussion);

Still to come:

1. Deeper refactoring of plot_sbc_ecdf() and plot_sbc_histogram();
2. Make module inputs compatible with nested sample dictionaries (discussion focus for above point 5);
3. Systematic testing of all refactored modules;
4. A user-friendly workflow.diagnose() front-end (or the like) that encapsulates the modularized design of the modules.

Any feedback or suggestions are welcome.

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[LarsKue]

2000+ lines in 2 commits is too much, you might have to redo this one