Alterations to allow Blatt-Weisskopf factor for parent to be created in a model with only a K-matrix

Summary: Updates to LauBlattWeisskopfFactor and LauResonanceMaker to allow a model to be generated only containing a K-matrix (for visualisation purposes)

Test Plan: Visualising the K-matrix in B -> DDh Dalitz analyses

Reviewers: tlatham

Reviewed By: tlatham

Tags: Laura

Differential Revision: https://phab.hepforge.org/D52