Add switch to force allocation of all exports

[tclune]

This functionality is desired for testing gridcomps in isolation. The switch will avoid the need to artificially connect all exports to imports in some other gridcomp.

Longer term we may want to allow VariableSpec to specify that any given export should be allocated regardless of connection status. E.g., the export might be used for computing other exports. OTOH, this could also be handled via "export dependencies" which already have some implementation foothold in MAPL3.