**As** an OpenProject user
**I want** to calculate custom field values based on other attribute fields
**so** that I can automatically calculate values.

Acceptance criteria:

* Add option to Begin of the insertionaEnd of the insertion custom field (e.g. separate custom field type) to infer value from other attributes.
* Support basic Begin of the insertionoperations:
* Addition,End of the insertion Begin of the deletion operation (e.g. addition,End of the deletion subtraction, Begin of the insertionmultiplication, division
* Allow use of parentheses (or similar) to adjust the order of operation for addition/subtraction vs. multiplication/division

Out of scope:

* Exponentiation, median, variance, percentile, ...End of the insertion Begin of the deletion multiplication)End of the deletion