Foreach adapt

[gonidelis]

Adapt for_each in order to support for the begin and end iterators to be of different type

[jenkins-cscs]

Can one of the admins verify this patch?

[gonidelis]

Should I use the is_sentinel_for trait too?

[msimberg]

add to whitelist

[gonidelis]

Important Note: Please don't merge yet. Need to add a couple of doxygen comments.

@hkaiser Please tell me if HPX_CONCEPT_REQUIRES_ format is fine:

• The requirements are all aligned
• The && operator is separated with space
• There is extra indentation for the second line of the "two-lines requirements"
• clang-format off includes the template parameters declaration (when it includes just the HPX_CONCEPT_REQUIRES macro, clang-format messes up the alignment of clang-format off and clang-format on comments)

[hkaiser]

LGTM otherwise, thanks!