Externally specified attributes

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As discussed in #470 it is useful to have information stored in program via a number of attributes. One risk is of course the TCL script problem, but I think here we do have a meaningful amount of information that should be specified somehow and would be too inflexible if specified directly within the filament source. One possible alternative would be to specify this information directly as attributes in the same file that #439 uses to specify gen parameters as well as toplevel component parameters (#440) so at least everything is together.

Just dumping this as an idea that goes well with the existing #441 ideas, but it also suffers from a number of the same problems, namely

1. Confusion from external file information overriding things in source code
2. Too much externalized information