XTCE12 — Add raw units to ParameterType

A specific syntactic element to capture raw units if it needed, possibly in addition to engineering units is not provided. There is a unit set in XTCE but was not intended to be used in this manner. Reported by JPL.

Allow for specification of the unit form

Propose to augment the UnitType for the Unit element to permit another optional attribute for the form of the units.

Add a new simpleType to restrict the new attribute:

<simpleType name="UnitFormType">
<annotation>
<documentation>Optionally specify if this information pertains to something other than the calibrated/engineering value.</documentation>
</annotation>
<restriction base="string">
<enumeration value="calibrated"/>
<enumeration value="uncalibrated"/>
<enumeration value="raw"/>
</restriction>
</simpleType>

Update the annotation for the UnitType complexType, add annotation to the description attribute, and add the
optional form attribute:
<complexType name="UnitType" mixed="true">
<annotation>
<documentation>Describe the exponent, factor, form, and description for a unit. The unit itself is in element Unit in UnitSet. See UnitSetType. The attributes are optional because different programs use this element in different ways, depending on vendor support.</documentation>
</annotation>
<attribute name="power" type="double" use="optional" default="1"/>
<attribute name="factor" type="string" use="optional" default="1"/>
<attribute name="description" type="string" use="optional">
<annotation>
<documentation>A description of the unit, which may be for expanded human readability or for specification of the nature/property of the unit. For example, meters per second squared is of a nature/property of acceleration.</documentation>
</annotation>
</attribute>
<attribute name="form" type="xtce:UnitFormType" use="optional" default="calibrated"/>
</complexType>