release-1.6.8

this is a point release in the stable series. the sha256sum is

b676f81bd8cc661a8f76e03ad449da4444f770b6bec3e9accf013c636f690905 darktable-1.6.8.tar.xz
ec4b1ad797ea7a483d7fc94724de99a1d18da7d7f75071220e1d313e0a4d8a53 darktable-1.6.8.dmg

and as always, please don't use the tarballs provided by github (marked as "Source code").

changes:
clipping, santiy check for custom aspect ratios
read lensmodel from xmp
handle canon lens recognition special case
general cleanups

rawspeed:
Canon EOS M3
Canon EOS 5Ds (R)
Nikon 1 J5
Panasonic DMC-G7 (4:3 aspect ratio only)
Fujifilm X-T10
Pentax K-S2
Panasonic TZ71
Olympus TG-4
Leica VLUX1 4:3 aspect ratio mode

adobe_coeff:
Canon EOS M3
Canon EOS 5Ds (R)
Nikon 1 J5
Panasonic DMC-G7
Fujifilm X-T10
Pentax K-S2
Olympus TG-4

wb_presets:
Samsung NX500
Panasonic TZ71

noise profiles:
Sony ILCE-5100
Fujifilm HS50EXR
Canon EOS 5Ds R