update density scale defaults

[malcolmbarrett]

• add failing tests for the Laplace scale estimator and the shared scale seam
• estimate the Laplace scale under the Laplace through a shared scale seam
• add failing tests for the t scale default and the numerator model spread
• estimate the t scale under the t by default and read a numerator model at its family's spread
• add a Laplace bootstrap arm and name the estimator the heavy-tailed block tests
• document the scale estimator each density family reads its spread at