#2119 Add IndexFeatures to CustomKernel and add document.
Fix comments and ERROR information.
Fixed indentation and removed copy function.
Fixed travis bug, EXPECT_NEAR needed.
Merge pull request #2155 from karlnapf/develop

added new in-place interface for adding subsets
Fix path for data set of variational_approx_example.cpp
Merge pull request #2157 from karlnapf/develop

fixed doc typos
Merge pull request #2122 from pl8787/indexfeatures

#2119 Add IndexFeatures to CustomKernel
potential fix for ref-counting bug in CList. Method delete_element() did not de-crease ref-count