Gtplus merging grappa

Aim: to move grappa functionalities out of gtplus and move it into mri_core

1. add 2D and 3D grappa functions to mri_core
2. add mri_core_definition.h, moving definitions out of gtplus
3. add mri_core_utility to store useful utility functions
4. clean many functions from gtplus util

To do:

1. add a sum_over_dimension function for hoNDArray, replace all current sumOver functions with this general one
2. replace multipleMultiply with the one in hoNDArray_elemwise.h
3. move the coil map estimation functions from gtplus
Don't understand why this is a class??