FormatTools: add null check to getScale

In the getScale method, if the specified AxisType is not present in the
given Metadata, return a default 1.0 instead of throwing NPE.
FormatTools: add calibrate(Axis) method

Added a calibrate(CalibratedAxis, scale, offset) method to FormatTools,
as previously you could only calibrate ALL axes in a Metadata.

These methods still provide the convenience of checking for LinearAxis
types.