Faster convergence in both derivatives

If we rotate and jitter the grid points we can get some faster convergence
for the derivatives uy and vx, but loose some convergence for ux and vy.
However, if we just rotate the grid points we loose the faster convergence
for the derivatives in the divergence-free condition and gain faster
convergence in other directions (the x and y axis rotated).

More test are necessary to understand why we get this faster convergence,
principally when we have some random distribution of points.
3D kernel

Add a script to check that the divergence of the columns of the 3D kernel
is in fact zero.
Divergence-free in 3D

A folder for the 3D divergence-free interpolant was created. The core
function rbfDivFreeMatrix3d.m was added.
Moving file to correct folder
Completing the move of the file
3D divergence-free script and functions

Add 3D divergence-free script and functions. Still testing them.
3D version working
Adding important routines to 3D case

Add Gregory Fasshauer's routines for RBFs.
Add some plotting features

Change the way we are plotting the errors decay for the derivatives of the
divergence-free interpolants (2D and 3D case).
Updating plotting codes for figures
Test functions for 2d case with random grid

Adapt the test functions to work with a random distribution of grid
points.
Divergence-free interpolants on randomGrid

Now we can do test with random grids for divergence-free RBF and
polynomial interpolants. This commit changes the FD_DivFreeMatrixStream,
which breaks other functions and scripts that depends on it.
nearstNeighbors function

Adding nearstNeighbors function that I forgot to commit on previous a
previous commit. This functions finds the nearst neighbors of a given
point of a given data set.
Better plotting for divFree polynomial FD

Add latex to the legends and labels of the plots of the script
testConvergenceDerivativeStream.m. Also include square grid for easy
switching between random grid and square grids.
Performance improve for RBF_DivFreeMatrix

Modify RBF_DivFreeMatrix for better performance. Not calculating
anything symbolic that is not necessary.
Update in testConvergenceDerivativeRBFFD

- Plotting using the latex interpreter;
- Adapted to use the performance improvement of RBF_DivFreeMatrix;
- Add code for squared structured grids.
Ajusting title size of plots
Better plotting and random grid switch

Add better features to the plots and add a switch variable to change
between structured grids and random grids.
Small bug and publish typo fixed

Calculating filled distance correctly and small typo fixed.
3d version with better plots

Changed the code a bit to use the nearstNeighbors function in case we
want to use unstructured grids later. The plots follow the patter of the
2d version.