Base asset class

Base asset class, which is used to add the first basic equations for the general assets. These are equations to:

1. Set the temperature or pressure to a given value.

2. Set the pressure or temperature equal to the pressure of the connected node.

This class is split from the base item class to avoid import errors. This class needs to import the node class for type checking, while the node class needs to import the asset class. This resulted in a circular import error. By splitting this class, this is avoided.