Support conditional statements in @mtkmodel

[ven-k]

This PR adds support for

• Conditional default values

@mtkmodel A begin
    ...
    @parameters begin
        a = condition ? 1 : 0
    end
end

• Conditional equations and components inside the respective blocks
  • Inside @equations
  • Inside @components
  • Inside @parameters and @variables

@mtkmodel A begin
    ...
    @components begin
       if condition1
            b = B()
       end
    end
    @equations begin
       if condition2
            a ~ 0
       end
    end
end

• If branch at top level
  • Equations
  • Components
  • Parameters

@mtkmodel A begin
    ...
    if condition
        @components begin
             b = B()
        end
        @equations begin
                a ~ 0
        end
    end
end

• Adds relevant docs.

[ChrisRackauckas]

Should we also generate events here or lower @YingboMa ?

[baggepinnen]

I guess it depends on whether condition is a structural parameter or a symbolic expression. It'd be nice to support both, since structural parameters are often used to determine which equations to include.

[ven-k]

Bump

[YingboMa]

The MTKStdlib failure looks real. Also, could you merge master back to this branch and run formatter?

[ven-k]

@YingboMa I've rebased and have fixed issues that threw error in MTKStdlib tests. This can be reviewed.

[YingboMa]

This looks really good!