New class 'Engine' responsible for both training sequentially and non-sequentially.
Currently training with old MetaModEngine and in MetaMod directly still possible, but will be deprecated and removed soon.
Also started splitting ExpDesign into InputSpace (for MetaMod, created dists and polynomials) and ExpDesign (for Engine, can also sample)

Started testing MetaModel while also refactoring a bit more by moving code between ExpDesign and MetaModel

Added tests for Inputs and started on those for ExpDesign.
Regarding the second there are some open questions noted in issue #32

[surrogate] fix bugs in the PCE implementation of the Rosenblatt transformation for dependent variables.

[surrogate] fix bugs in the implementation of the Rosenblatt transformation for dependent variables.

[surrogate][reg_fast_laplace] handle variance zero and start from a single basis vector with the largest projection on targets instead of bias.