Implemented serialization and deserialization of models in symbol fields
through the SIM_VALUE class. We don't carry the Spice legacy of
case-insensitive suffixes, instead we conform to the SI standard (i.e. M
is Mega, not milli, P is peta, p is pico).
Parameter grid value validation is implemented by simply not allowing
any characters that will make the value invalid (instead of highlighting
the field in a red color). This will likely be changed at some point in
the future.
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