Now DCPSE-related unit tests which require EMatrix (and therefore Eigen) are compiled only if EIGEN3 is available, to avoid build failures on systems where EIGEN3 is not installed.

Apparently in gcc-5 the abs function has return type int instead of double, switching it to fabs solved the issue.
The visible symptom of the issue was getting "nan" values in the computed differential operator values in the Dcpse tests.

Fix for failing build on CI ubuntu machine due to not supported implicit construction of returned Point object.

Fix symbolic derivative computation in Monomial and restructure code to make it suitable to final integration into DCPSE.

1) The derivative computation in Monomial was totally wrong as it did not account for the scalar factor. Now it has been fixed and the tests modified accordingly.
2) The interface of Support has been changed to make it more suitable for the expected usage in the integrated DCPSE computation. Interfaces of the other classes interacting with Support has been changed accordingly.

The DcpseRhs class provides machinery for getting the RHS to be used in the linear system associated with DCPSE computation. Analogously to Vandermonde, the user has to initialize the class with the MonomialBasis and differential signature in use and then pass an existing vector object to be filled to the getVector() method.
The fix in CMakeLists.txt is to allow MacOs and CentOS builds to be successful: in these cases the EIGEN package may not be found at first and this case is now supported.

In cases when alpha_min was 1, the case with all exponents equal to 0 was skipped, while it is necessary in any case.

Update pointer to openfpm_data submodule to include fix for grid_key_dx and removed print statement from test.

The Support class allow getting the required number of neighbouring points of a given one.
It does so in a way that only expands the search area in directions where we are performing derivation.
Now linking to a newer version of openfpm_data as a fix in const-ness of some operators was required.
Also minor fixes to some other tests.

Possibility to get derivatives of Monomial and MonomialBasis objects has been added to allow for easy arbitrary derivative computation when building the RHS in the DCPSE linear system.
Monomial was also renamed from MonomialBasisElement.

Add check for right dimension of the set of neighbouring points w.r.t. the size of the monomial basis.

The Vandermonde matrix builder fills an EMatrix using a given multivariate monomial basis, the point the matrix belongs to and a set of neighbouring points.
Initial version of unit tests are present.

MonomialBasis allows generating the right exponent multi-indices to be used to build the Vandermonde matrix for DCPSE.
Updated openfpm_data submodule is necessary for a fix included there.