Provide an explicit list of constructs which are legal in Base Modelica but not in Modelica

[casella]

Base Modelica is mostly a subset of Modelica, with some exceptions, which can lead to code that is valid Base Modelica but not valid Modelica.

This is of course a crucial issue for implementors, particularly those who already have a working Modelica compiler:

• what is allowed in the Base Modelica syntax but isn't in Modelica?
• what additional semantics may Base Modelica have that Modelica doesn't

This information is somewhat scattered in MCP 0031, but this is not really convenient if one needs to take design decisions early on. I believe we should list those differences explicitly in the MCP.

[casella]

Adding @perost and @adrpo to the loop.