BUG: Inconsistency detected by ld.so due to static const string

Change-Id: I56202d38db4deff8b63f58b7606127050842ca55
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
DOC: CreateTestDriver srcs should have the .cxx extension.

CMake can have a hard time finding the relative path without them.
The extensions are used for the ITK tests drivers.

Change-Id: I1cd1cb95fdb484d732540a77755f61cc7ca88bfc
Merge topic 'TestDriverDoc'

8fa6806 DOC: CreateTestDriver srcs should have the .cxx extension.
Merge topic 'FillPerformance'

ca474f5 PERF: Use std::fill for image filling
Merge topic 'ld-so-inconsistency-watershed'

6c8c940 BUG: Inconsistency detected by ld.so due to static const string