Add math constants like pi and e

Similar to the math expressions like "sin" mentioned under "9.2.1 Operators", it is very helpful to be able to use constants like "pi" or "e" within OpenSCENARIO XML. "pi" is a very common candidate, especially in conjunction with the already introduced trigonometry operators such as "sin". We already added support in our implementation to be able to use the expression in the NCAP scenarios https://github.com/vectorgrp/OSC-NCAP-scenarios. I think, @emil.knabe already added it to esmini as well. It should be included in the standard, possibly under a separate sub-chapter "9.2.2 Constants"