Custom variables for use in math expressions

[tedchwang]

Is your feature request related to a problem? Please describe.
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An organization may have a large number of variables that are unsuitable to become model nodes, because they would clutter the model and slow down computation. Example: 1000 products each with their price, inventory numbers, sales velocity, return rate.

Describe the solution you'd like
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JSON objects that a user can CRUD on the UI, or import as a TSV file would be nice. Also consider possibility to update with API in the future. The properties of the object should be accessible in formulas in a model node, and in numerical fields of an action. It'd be good to be able to calculate these variables with a formula, too. Such as the inventory level of a product changing with stock input and output.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
I've considered creating the variables in the model and grouping them to reduce clutter, but the manual labor would be error-prone and prohibitive for organizations with many individual items to track.

Additional context
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