DeepXDE v0.5.0

Areas of improvement

• PDE, TimePDE, and IDE allow the reference solution to be None
• postprocessing: Add 3D plot
• Bug fix: Replace net.x with net.inputs
• Refactor: move feed_dict from Model to Map
• Update docs

API changes

• data.OpDataSet supports whether standardize
• maps.MfNN supports train only high or low fidelity network

New APIs

• maps.OpNN supports setting default function input