Revisit CNPs

[EdwinB12]

Prerequsities:

1. Have a way of measuring Uncertainty Quantification.
   • We now have Add calibration plot #897 and Add CRPS metric #904 to help measure this performance.
2. Find a dataset where gaussian processes struggle

[sgreenbury]

See https://github.com/wesselb/neuralprocesses for comparing / replacing previous impl (CNPs, attentive CNPs).