Merge pull request #900 from karlnapf/master

Fixed a serious CustomKernel subset related bug
Fixed Factor Analysis integration

- Fixed bug with unset features
- Added Factor Analysis to modular interfaces
Handled case of exceeding number of landmarks in landmark mds
Updated tapkee library

- Better parameters handling: proper invariants to avoid bugs
- Better exceptions usage
- Better naming of types
updated documentation for kernel MMD two-sample testing