PERF: Use std::fill for image filling

SimpleITK's Image constructor initializes an image to be of all
zeros. The performance comparison was done with this SimpleITK
allocation and filling a 512^3 image is the median over 50 runs, in
seconds:
       fill   old
-----------------
uint8  .087  .227
uint16 .092  .186
float  .196  .273
double .392  .438

Change-Id: Iec226a1d875b0f35f409883a42f659160d10ac42
Merge topic 'FillPerformance'

ca474f5 PERF: Use std::fill for image filling