Move unnecessary include from header to source file.
Merge pull request #2219 from iglesias/develop

Move unnecessary include from header to source file.
added first draft of linalg unifying backend
added cmake options for setting linalg backend libraries
added viennacl support and redesigned linalg policy
added hard requirement for linalg backends cmake options
added explicitly set eigen3 backend dot unit-test
added viennacl dot and made it easy to call explicit backend dot
Merge pull request #2217 from lambday/feature/linalg

Linear algebra unifying backend (WIP) - First draft