CoefficientsType should require at least one Numerator

[crotwell]

Mathematically, a Coefficients filter has to have at least one numerator, which can be 1. Otherwise the output time series would be all zeros.

A no-coefficient Coefficient filter might be used to just be a container for units, but this should be discouraged. Requiring at least one numerator would force using of at least a single coefficient, even if it is 1.

Change:

<xs:element name="Numerator" minOccurs="0" maxOccurs="unbounded">

to

<xs:element name="Numerator" minOccurs="1" maxOccurs="unbounded">

Similarly a FIR filter should have at least one NumeratorCoefficient so change

<xs:element name="NumeratorCoefficient" minOccurs="0" maxOccurs="unbounded">

to

<xs:element name="NumeratorCoefficient" minOccurs="1" maxOccurs="unbounded">

[rcasey-earthscope]

Seems reasonable to me.

[crotwell]

Related, docs currently say "Laplace transforms or IIR filters can both be expressed" with Coefficients, but I think they meant "analog filters" instaed of IIR filters. It makes sense to me to represent a digital IIR filter with coefficients, especially if implemented in the time domain.

See PR #126 for doc fix for this, but not schema change.