Contrast enhancement corrections

See merge request !18

Fix clang missing override warnings

See merge request !30

Refactor logging for the whole library

Closes #1515

See merge request !20

STY: Change ITK_OVERRIDE for override in whole code (find Modules -type f -exec sed -i 's/ITK_OVERRIDE/override/g' {} +)

DOC: Create a new chapter with extended filenames and environment variables in cookbook. Document all environment variables

Parameter bool

See merge request !15

App engine flags

See merge request !8