Gftool

New Features
* Officially support Python 3.10
* Add retarded-time Bethe Green's `~gftool.lattice.bethe.gf_ret_t` function
(6ffc7c91)

Internal improvements
* Switch from Travis to GitHub actions 20 (23ba0a34)

- This adds test for Mac and Windows

Documentation
* Fix various errors using [Velin](https://github.com/Carreau/velin)
(03ff6c8e)

Bug fixes
* Accept singular constrains in `~gftool.linalg.lstsq_ec` (167e7886)
* Accurately calculate `gftool.lattice.sc.dos` around ``eps=0`` (5693184e).
Previously, DOS was incorrect for tiny values, e.g. ``eps=1e-16``.

Breaking Changes
* Drop support for Python 3.6, **minimal version** is now **3.7**
* Content of `gftool.matrix` was renamed more appropriately:
- `xi` of `~gftool.matrix.Decomposition` is now
`~gftool.matrix.Decomposition.eig`, as it contains the eigenvalues
- New functions
`~gftool.matrix.decompose_mat` for general matrices,
`~gftool.matrix.decompose_sym` for complex symmetric matrices,
and `~gftool.matrix.decompose_her` for Hermitian matrices.

Depreciations
* Deprecate the `~gftool.matrix` functions
`~gftool.matrix.decompose_gf`,
`~gftool.matrix.decompose_hamiltonian`,
`~gftool.matrix.Decomposition.from_gf`,
and `~gftool.matrix.Decomposition.from_hamiltonian`.

Documentation
* New index page independent of README,
separated :doc:`getting-started` page.
* Improve :doc:`tutorial` and `gftool.matrix`
* Generate PDF documentation on ReadTheDocs (3122e1ba)

Internal improvements
* Use eigendecomposition instead of SVD in `gftool.beb` (0475c110)
* Drop slow `~numpy.asfortranarray` in `gftool.matrix` (4865cc05)
* Use `pre-commit` (6f4028d3)

Bug fixes

CPA:

* return scalar `mu` in `gftool.cpa.solve_fxdocc_root` (10fae4d)
* find `mu` more reliably

Other New Features

* SIAM: add greater and lesser Green's functions
`~gftool.siam.gf0_loc_gr_t` and `~gftool.siam.gf0_loc_le_t` (ea541f3)

New Features

Implement `gftool.cpa` and `gftool.beb` to treat disorder (c3bad20c)

New Features

The 3D cubic lattices were added:

* body-centered cubic `gftool.lattice.bcc` (406acef8)
* face-centered cubic `gftool.lattice.fcc` (ddd559cb)

New Features

The `gftool.lattice` module was extended, especially regarding two-dimensional lattice.
There where also some enhancements, given DOS moments are now up to order 20,
and they should be accurate to machine precision.

The following lattices where added with full interface:

* Simple cubic: `gftool.lattice.sc` (4e30210) by aostlin
* Honeycomb: `gftool.lattice.honeycomb` (7aa3133)
* Triangular: `gftool.lattice.triangular` (c56f33e)

Local Green's function and DOS is now also available for the following lattices:

* Lieb: `gftool.lattice.lieb` (c76e948)
* Kagome: `gftool.lattice.kagome` (28a41c0)
* Bethe lattice with general coordination: `gftool.lattice.bethez` (2648cf4)
* Rectangular: `gftool.lattice.rectangular`

Other New Features

* add retarded time Green's function give by its poles `gftool.pole_gf_ret_t`
* added `gftool.siam` module with some basics for the non-interacting siam

Depreciations

* `gftool.density` is deprecated and will likely be discontinued.
Consider the more flexible `gftool.density_iw` instead.

Documentation

* Button to toggle the prompt (>>>) was added (46b6f39)

Internal improvements

* Ensure more accurate `numpy.sum` using partial pairwise summation for
generalized ufuncs (2d3baef)