Add support for defining custom functions, add all built in functions

[jClugstor]

BaseModelica has a number of built in functions, listed here https://github.com/modelica/ModelicaSpecification/blob/MCP/0031/RationaleMCP/0031/functions.md
We should add them.
We will probably be able to implement these functions in Julia, or maybe we'll have to look for existing implementations for specific functions.

Also, BaseModelica supports custom defined functions. We need to be able to read these functions and use them in the model equations. We'll probably use symbolic function registering to be able to use them in MTK.