COMP: Allow single precision space definition

The spacing and physical space definitions for
ITK images is hardcoded as double precision.  This patch
allows for an advanced compiler option to be provided
that rebuilds the entire toolkit so that physical
space is represented by single precision floating
point numbers.

The primary reason for this is to test for performance
issues that arise due to implicit conversion between
double and float.

Change-Id: Ic65146abc390240765682b0f74df60e55061fd2c
Merge topic 'SinglePrecisionSpacingCapabilities'

bc929a9 COMP: Allow single precision space definition