Allow user to select component or module to open when there is more than one root object

[geoffbaldwin]

When a combinatorial design is saved to SBOL, only one of the various permutations are visible when the SBOL file is opened in SynBioHub. SBOL Designer has an option as to which of the combinatoric designs is saved, but this is not currently implemented in SBOL Canvas.

[cjmyers]

When in the Module Editor and import an SBOL file, if there is one root ModuleDefinition, you can open it. If there is more than one, then provide a list of root ModuleDefinitions to select from. If no root ModuleDefinitions, tell the user there are no MDs in the file.

When in the Component Editor and import an SBOL file, if there is one root ComponentDefinition, you can open it. If there is more than one, then provide a list of root ComponentDefinitions to select from. If no root ComponentDefinitions, tell the user there are no CDs in the file.