Wimprates

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* Change default v_0 and v_pec to match [current conventions](https://arxiv.org/abs/2105.00599) (#14)
* If no time is provided, spectra are now calculated at a reference time (14)
* Fix bug where user-specified halo models would not override v_0 (14)
* Fix tests for numpy 1.24 (15)

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* Restore python 3.7 compatibility (13)

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* Fixes for alternate materials (7)
* Update notebooks, continuous integration tests (9)

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* Fix technical release issue

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* Alternate materials for SI scattering (4)
* Faster J200 timestamp conversion (5)

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* Flexible halo model (3)
* DM form factor choice for DM-electron scattering