ENH: Remove double underscore in header guards (.txx & __otb*_h form)

Used the following command:

find . \( -iname *.txx -and ! -path *ThirdParty* \) -exec perl -pi -w -e 's/__otb(.*)_txx/otb$1_txx/g;' {} \;
find . \( -iname *.h -and ! -path *ThirdParty* \) -exec perl -pi -w -e 's/__otb(.*)_h/otb$1_h/g;' {} \;

Fixes many, but not all, clang -Wreserved-id-macro warnings.