* Add parallelization to Chi2 fitting.

- Adapt vectorized TF1 for a Fitter with fixed parameters.
- Made the Chi2FCN Evaluating loop independent of the size of the SIMD array.
- Parallelized both vectorized and scalar versions of the Chi2FCN evaluation.
- Added new fExecutionpolicy field to the Chi2FCN class that will determine the policy to use for the chi2 function evaluation: multiprocess, multithread or serial(none).

* Provide automatic chunking in parallel Chi2 Fit

* Change naming in template resolver

* Fix error calling EvalParVec with ftype=1

* Template Paramfunctor

Templates Paramfunctor and adds const parameters interfaces for evaluation.

* Add vectorized Chi2 Evaluation

* Add Functor type resolving to TF1

* Vectorize and parallelize unbinned likelihood fit

* Introduce the Double_v type

* Linked MathCore with Vc

* Prepare for chunking

* Adapt Fitter interfaces for vectorization

* Improve use of VecCore

* Change Mathcore linking from Thread to IMT

* Allow the evaluation of Chi2 with coord errors

* Improve identation

* Disable the multiprocessing of the fitting

* Cleaned remaining Vc directives

* Link vecCore instead of Vc

* Define R__HAS_VECCORE

* Differentiate VecCore code at compile time

* Fix warning due to unused args

* Restore original conditions for Chi2Evaluation

* Make fFunctor transient again

* Reenable Chi2Gradient and Chi2Residual evaluations

* Add ValuePtr documentation

* Fix white lines between functions

* Fix Chi2 evaluation of multidimensional data

* Fix Memory alignment in the vectorized Likelihood fit.

This reintroduces Vc dependencies

* Fix for classic builds

* Conditionate MathCore linking

* Only Fit Multithreaded if IMT is enabled

* Fix multidim integral error in the Chi2 fit

* Fix LogL evaluation of multidimensional unbinned data

* Improve VecCore usage

* Simplify fFunctor check expressions

* Move the Double_v type into the ROOT namespace

* Make executionPolicy an enum

* Explain functor operator resolving in TF1

* Improved TF1::fType documentation

* Adapt execution policy option of the histogram fit

* Completed move of executionPolicy to a enum

* Add tests for the new parallel fitting.

This tests check both Chi2 and Unbinned LogL fitting in each one of the
execution policies available.

* Correct function naming

* Rename global variables to supress shadowing warning.

* Remove legacy parallel implementations