Implement Testcases for Davidson

[JensWehner]

Within the Catch2 framework we need small testcases for
the Symmetric Davidson case.

They have to cover the following aspects:

datatypes: float, double, std::complex, std::complex
selection rules: min and max

and the following error conditions:
number of iterations exceeded without convergence,
input of the already diagonlized matrix,
some sort of numerically unstable problem(degeneracy)

and validation of input parameters:
too large searchspace
too many eigenvalues requested.

it has to cover restarts/deflation

reference values can be obtained from the Eigen solvers for dense matrices.

one test case should include a user defined matrixproduct.

[v1kko]

We can use TEMPLATE_TEST_CASE (or even fancier, depending on catch2 version) to quickly enable support for various types in our tests : https://github.com/catchorg/Catch2/blob/master/docs/test-cases-and-sections.md#type-parametrised-test-cases