Operation to perform arbitrary dataset math

[forman]

Expected behavior

Cate's Python API allows users to easily add new operations to Cate, either adding it directly to Cate's code base or by writing a plugin, that contributes operation registrations to Cate.

It would very beneficial if users of the GUI could also easily and interactively create operations that perform arbitrary dataset math and output a new dataset with computed variables.

Like any other operations, these user-defined operations would be stored in the workspace's workflow. A library of code snippets could help users not so familiar to programming in getting started.

Specifications

Cate 1 and 2-dev1

[forman]

Implemented in 2.0.0-dev.3:

• operation compute now has an input annotation script_lang='python' on its script input.
• the GUI recognizes that and can bring up a code editor with syntax-highlighting (react-ace editor)