DOC: Fixed Doxygen description BoundingBox Get member functions
DOC: Fixed Doxygen description BoundingBox Get member functions

The documentation of `BoundingBox` member functions `GetCenter()`,
`GetMinimum()`, and `GetMaximum()` incorrectly stated that these
member functions might return a `nullptr`. This commit uses the term
"a point at the origin" instead.

Also mentioned that `GetCorners()` is a legacy member function.

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