Remove zero start value and explain alternative.

[HansOlsson]

Remove the default start-value of 0.0 (and similarly for other types).

The reasons are:

• It is not necessarily consistent with min/max - which just causes unnecessary problems. In case some value is needed that is now handled.
• For parameters it is problematic as the current rule https://specification.modelica.org/master/equations.html#initialization-initial-equation-and-initial-algorithm indicates that modified start-values are special. This is somewhat weird in itself (when is start set but not modified?) but with the MCP for removing modifiers it would become a complete mess. Instead it can now state that it uses the value for the start-attribute if it has any.