Allow to evaluate derivative of a discrete global basis function.

Added:
* Struct defining types for function range and Jacobian.
* A new specialization of FlatVectorBackend.
* A simple test case taking derivatives of scalar and vector valued functions.
Status Job ID Name Coverage
  Test
passed #14150
debian:10 gcc:c++17

00:09:51

passed #14153
debian:8--gcc

00:09:24

passed #14152
debian:9--clang

00:09:50

passed #14151
debian:9--gcc

00:09:47

passed #14155
ubuntu:16.04--clang

00:09:11

passed #14154
ubuntu:16.04--gcc

00:09:13