Add framework module and curve_analysis module

[chriseclectic]

Summary

• Moves BaseExperiment, BaseAnalysis, ExperimentData to the qiskit_experiments.framework module.
• Moves the composite folder to the qiskit_experiments.framework module, ParallelExperiment and BatchExperiment should be imported from here now.
• Adds basic documentation for creating experiment and analysis subclasses to the framework module documentation
• Renames the analysis module to curve_analysis
• Adds a table for modules to simplify formatting of qiskit_experiments.__init__.py.

Details and comments

TODO as a follow up is reviewing and re-organizing the contents of the curve_analysis module.

[eliarbel]

Hi Chris, I think we should put all the framework classes together, that is the BaseExperiment and BaseAnalysis classes together with the composite directory classes. Although the composite classed won't be derived from (hence maybe we should rename base to framework) I still find it odd that we have a dedicated folder for the composite classes as I consider the base and composite classes as meta-classes in the experiments package