DeepXDE v0.12.0

This release is mainly about DeepONet.

API changes

• Rename OpNN to DeepONet
• Rename OpDataSet to Triple

New APIs

• Add dde.__version__
• Add data.TripleCartesianProd, maps.DeepONetCartesianProd, and maps.FourierDeepONetCartesianProd
• Add new metric: mean_l2_relative_error

Areas of improvement

• Bug fix: change 'sobol' to 'Sobol'