ACG histograms: use more general is_assignable instead of is_same.

Cf. MR !186
7 jobs for propvis-histogram-mr in 91 minutes and 34 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #17219
Apple
Mac-Cpp11

01:06:46

passed #17218
Qt551 VS2013
VS2013-Qt-5.5.1-x64

00:19:36

passed #17217
Qt56 VS2015
VS2015-Qt-5.6.0-x32

00:21:06

passed #17216
Qt56 VS2015
VS2015-Qt-5.6.0-x64

00:17:55

passed #17214
MultiThreads Linux
clang-c++11-Qt5

00:09:30

passed #17215
MultiThreads Linux
cppcheck

00:36:54

passed #17213
Linux
gcc-c++11-Qt5

00:06:30