Sfs

* build doc fix, use sphinx4, mathjax2, html_css_files

* New default driving function for `sfs.td.wfs.point_25d()` for reference curve

* New function `sfs.fd.source.line_bandlimited()` computing the sound field of a spatially bandlimited line source
* Drop support for Python 3.5

* Switching to separate `sfs.plot2d` and `sfs.plot3d` for plotting functions
* Move `sfs.util.displacement()` to `sfs.fd.displacement()`
* Switch to keyword only arguments
* New default driving function for `sfs.fd.wfs.point_25d()`
* New driving function syntax, e.g. `sfs.fd.wfs.point_25d()`
* Example for the sound field of a pulsating sphere
* Add time domain NFC-HOA driving functions `sfs.td.nfchoa`
* `sfs.fd.synthesize()`, `sfs.td.synthesize()` for soundfield superposition
* Change `sfs.mono` to `sfs.fd` and `sfs.time` to `sfs.td`
* Move source selection helpers to `sfs.util`
* Use `sfs.default` object instead of `sfs.defs` submodule
* Drop support for legacy Python 2.7

* Driving functions in time domain for a plane wave, point source, and
focused source
* Image source model for a point source in a rectangular room
* `sfs.util.DelayedSignal` class and `sfs.util.as_delayed_signal()`
* Improvements to the documentation
* Start using Jupyter notebooks for examples in documentation
* Spherical Hankel function as `sfs.util.spherical_hn2()`
* Use `scipy.special.spherical_jn`, `scipy.special.spherical_yn` instead of
`scipy.special.sph_jnyn`
* Generalization of the modal order argument in `sfs.mono.source.point_modal()`
* Rename `sfs.util.normal_vector()` to `sfs.util.normalize_vector()`
* Add parameter ``max_order`` to NFCHOA driving functions
* Add ``beta`` parameter to Kaiser tapering window
* Fix clipping problem of sound field plots with matplotlib 2.1
* Fix elevation in `sfs.util.cart2sph()`
* Fix `sfs.tapering.tukey()` for ``alpha=1``

* Fixed metadata of release