add response filter type for non-response channels

[crotwell]

Define BaseFilterType to be used when response information is either unknown or not applicable. Idea is to positively state that information is missing, as opposed to just a missing element in xml.

Examples:

• Old channel where freq response is unknown, but maybe input and output units and overall gain are known. Possible this could be done with a polynomial.
• Not applicable like a log channel
• Missing, data exists but no response information is known, perhaps digitized paper record

See #16 for how done in documentation-only for ver 1.