Add tests for core, kernel, inf, lik, mean, and an example GP

[hadsed]

Pretty much the title. Add the commits where tests are added here.

[hadsed]

Core.py tests added: 28aa927

[hadsed]

Kernels.py tests added: 302fa8f

[hadsed]

Good tests for main GP class (will be merged with master soon, previous gp tests are not very reliable): https://github.com/hadsed/PyGPML/blob/no-matrix/testing/test_gp.py