Make heatmap and panel APIs and actions more consistent with one-another

[GilesStrong]

Currently the two classes have similar init args, e.g. init_xyz and init_xy_span, however they can have different conceptual meanings, e.g. init_xy_span in Panel is a 2-tuple of floats specifying the panel dimensions in x and y, but for DetectorHeatMap it is 2-tuple of floats specifying the range in both x and y within which the panels are expected to be found, and the a single width is automatically computed from this.

Additionally, Panel learns xy, z and xy_span using a Gaussian model, and the final parameters can be accessed directly from these. But DetectorHeatMap instead learns mu, sig, norm, and z and the user must interpret them into detector parameters. #112 will help with this.