ttools: careful about zero on histogram log Y axis

Use Double.MIN_VALUE rather than 0 for data base of histogram bars
when Y axis is logarithmic.  In fact 0 may work, but it depends on
the details of Infinity handling in surface graphics<->data
transformations, best not to rely on that.
ttools: filter out infinite data values in Axis

The Axis.dataToGraphics function now checks for log(0) and returns
NaN rather than -Inf in that case.  This is a bit questionable,
and may be changed in the future, but it avoids having to do quite
a few checks downstream.
ttools: add DataSpec.isCoordBlank method

Code can now tell up front if a column is known to be blank for all entries.
ttools: add linear fit plotter
ttools: implement weighting for linear fit layer
ttools: allow report information to be passed from plots back upstream

Drawing now has a new method getReport which can be used to interrogate
the plot plan for plotting information.  This means upstream control
components can have access to selected items of information generated
as part of plot layer generation.
ttools: actually pass reports back from histogram/linear fit plots

The new reporting framework is used by HistogramPlotter to pass back
populated histogram bins, and by LinearFitPlotter to pass back
fitting coefficients.
topcat: move LayerId class out of PlotPanel for re-use
topcat: provide plot reports from PlotPanel
topcat: pass plot report information back to LayerControls

At the moment the reports are just sent to a logger that writes
report information through the logging system.  But the hooks
are there to do something more sophisticated with it (I think).
topcat: count LayerControls with no layers as inactive

The ControlStackModel.getLayerControls method now includes
controls with no layers if the activeOnly parameter is false.
This makes more sense, and it's required to make ReportPanel
work properly.
topcat: display FormControl plot reports in the GUI

You now get a "Reports" panel at the bottom of the Form panel
where appropriate displaying the ReportMap entries generated by
plot layers originating from the Form in question.
topcat: document linear fit form in SUN/253
topcat: don't add plot2 window toolbar separator in superclass

Do getToolBar().addSeparator() in subclasses not StackPlotWindow
so that geom-specific plot windows can add custom actions to the
main toolbar block.
topcat: provide histogram export actions from histogram window

Actions are now provided in the plot2 Histogram window Export menu
to export the accumulated histogram bin data either to a new table file
or as a new table loaded instantly into the topcat application.

This reinstates functionality which was present in the old-style
histogram window.  It wasn't possible before now, because there
was previously no mechanism to pass the histogram bin data back
from the plotted layers to the GUI.  The new plotting ReportMap
framework now provides this.