FormatTools: do not default calibration to microns

There is absolutely nothing special about microns (um) that they should
somehow be the default unit for axis calibrations. Rather, the default
unit should be null -- i.e., unknown -- when no explicit unit is given.
FormatTools: fail loudly when axis is nonlinear

Previously, the calibration would fail silently. But in this scenario we
really want to know if something went wrong, including if the axis in
question is null.

Time will tell whether this change is too aggressive. If that proves to
be the case, we can perhaps return a boolean instead indicating success.
But doing nothing silently is bad news.
TIFFFormat: apply the spacing to the correct axis

Before this change, the spacing would be parsed but then lost, because
at the time of parsing, there are three axes: X, Y and TIME. There is no
Z, so a new Z is created. But later the axes get reshuffled and that Z
axis gets lost again, including its unit and spacing, in favor of the
time axis (which was never calibrated properly).

This change defers the assignment of the calibration until the axes have
settled down, resulting in a correctly calibrated Z axis.

This is not the greatest patch in the world, but it gets the job done
for now, until the ImageJ comment parsing logic gets rewritten as
documented in issue #203.

This change address the second half of Fiji bug 802.