Multibanding to reduce memory consumption

The multi banding procedure described in [arXiv:1604.08253](https://arxiv.org/abs/1604.08253) should be implemented, to reduce memory consumption.

From [arXiv:1712.08772](https://arxiv.org/abs/1712.08772): "_an adaptive frequency resolution Delta_f (f) is applied such that waveforms are sampled less densely at higher frequencies, where there is less power per frequency bin due to the faster frequency sweep (see Fig. 1). However, once a basis has been obtained, we up-sample by direct evaluation of the waveform model._".

----------

Look at [this package](https://pypi.org/project/gw-frequencies/#description) to implement it.