ENH: Refactor DoubleToString into NumberToString

NumberToString is now templated on value type. It resides in
Core/Common rather than in the ThirdParty module. This removes ITK
dependencies on ThirdParty.

This patch set allows a more generic implementation for converting
both floating point and fixed point numbers to ASCII versions that
are represented without numerical precision errors.

Change-Id: I38780ea1cca70965733e47bff6ac352a69522c97
Merge topic 'RefactorDoubleToString'

2bca5aa ENH: Refactor DoubleToString into NumberToString