TopicType and TopicStatus in BCF XML and BCF API differ in requirements

[GeorgDangl]

In BCF XML, TopicType and TopicStatus are marked as required:

• BCF-XML/Schemas/markup.xsd

  Lines 118 to 119 in bc48611

  	<xs:attribute name="TopicType" type="NonEmptyOrBlankString" use="required"/>
  	<xs:attribute name="TopicStatus" type="NonEmptyOrBlankString" use="required"/>

• https://github.com/buildingSMART/BCF-XML/tree/release_3_0/Documentation#topic

In the BCF API, they're optional. We should probably make it the same in both standards.