STYLE: Replace unsigned short by unsigned int as FixedArray index type
STYLE: Replace unsigned short by unsigned int as FixedArray index type

Replaced `unsigned short` by `unsigned int` as type of `index` parameter,
for both `GetElement(index)` and `SetElement(index, value)` member
functions of `FixedArray`. This improved consistency, as `unsigned int`
is being used throughout the entire definition of `FixedArray`.

Style improvements in the implementation of `FixedArray`:
* Fixed a typo in the documentation.
* Replaced `while` loops by range-based for-loops.
* Replaced `std::copy` by C++11 `std::copy_n`.
* Removed two dangling backslashes.

malaterre/ITK,vfonov/ITK,richardbeare/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,richardbeare/ITK,thewtex/ITK,LucasGandel/ITK,richardbeare/ITK,BRAINSia/ITK,hjmjohnson/ITK,BRAINSia/ITK,thewtex/ITK,Kitware/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,hjmjohnson/ITK,thewtex/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,BRAINSia/ITK,hjmjohnson/ITK,malaterre/ITK,LucasGandel/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,vfonov/ITK,thewtex/ITK,hjmjohnson/ITK,Kitware/ITK,thewtex/ITK,Kitware/ITK,richardbeare/ITK,richardbeare/ITK,LucasGandel/ITK,thewtex/ITK,BRAINSia/ITK,vfonov/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,LucasGandel/ITK,vfonov/ITK,malaterre/ITK,richardbeare/ITK,Kitware/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,BRAINSia/ITK,vfonov/ITK,vfonov/ITK,thewtex/ITK,BRAINSia/ITK,Kitware/ITK,hjmjohnson/ITK,BRAINSia/ITK,malaterre/ITK,LucasGandel/ITK,malaterre/ITK,vfonov/ITK