Pull request for CQML extension

[zaspel]

This pull request aims at adding the multi-fidelity learning approach called CQML, which is documented in the paper "Boosting quantum machine learning models with multi-level combination technique: Pople diagrams revisited":

• the core part of the new code are the new files qml/models/cqml2d.py and qml/models/cqml.py, which contain a 2d CQML implementation and a 2d/3d/nd CQML implementation. The 2d implementation follows the simplified idea presented in Section 3.4 in the paper, while the 2d/3d/nd CQML implementation is documented in Section 3.5

• examples/cqml2d_CI9.py is the code, which was used to create 2d CQML results on the CI9 data set in the paper

• examples/cqml_QM7b.py, examples/cqml_in2d*.py are the codes that were used to create the 2d/3d results for the QM7b data set in the paper

• the CI9 and the QM7b data set are included in the examples directory

[zaspel]

Since I worked on the commit history of my branch, I will close this pull request and open a new one, just to be safe!