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 Name Job ID Coverage
  Test
passed clang-c++11-Qt5 #17214
Linux MultiThreads

00:09:30

passed cppcheck #17215
Linux MultiThreads

00:36:54

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

00:06:30

passed Mac-Cpp11 #17219
Apple

01:06:46

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

00:19:36

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

00:21:06

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

00:17:55