Pybromo

**Changes:**

- Add support for Gaussian PSF (see 2).
- Rename function `hash_` to `hashfunc` to remove conflicts with variables names `hash_` in the code.
For user code, add a compatibility function `hash_` that raises a `FutureWarning`.
- Fix previously ignored `chunkslice` argument. This argument is either `'time'` or `'bytes'` and determines
if `chunksize` refers to the number of elements or bytes.
- Code and docstrings cleanups.

>This release was sponsored by Dr. Eitan Lerner of the Department of Biological Chemistry, in the Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem.

Main changes in this version:

- adds the option to save the particle's position for each emitted photon (timestamp).
- new section in the [smFRET simulation notebook](https://github.com/OpenSMFS/PyBroMo/blob/67b43e6f0cb1ceb165b4d7b229238edc83f2e2bf/notebooks/PyBroMo%20-%202.%20Generate%20smFRET%20data%2C%20including%20mixtures.ipynb) shows how to plot the PSF with an overlay of photon emission distribution.
- significantly improved docstrings and code cleanups
- refactor and few API additions, added an constructor `Particles.from_specs()` to more easily create multiple popualtions (see [notebook 1](https://github.com/OpenSMFS/PyBroMo/blob/67b43e6f0cb1ceb165b4d7b229238edc83f2e2bf/notebooks/PyBroMo%20-%201.%20Simulate%203D%20trajectories%20-%20single%20core.ipynb) for an example).

>This release was sponsored by Dr. Eitan Lerner of the Department of Biological Chemistry, in the Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem.