Bdsf

**NOTE:** the interactive `pybdsf` shell is no longer installed by default. Please see the [README](https://github.com/lofar-astron/PyBDSF#readme) for installation instructions.

* Update some functions as required by scipy versions >= 1.8.0 (PR 172)
* Fix build issues with Python 3.8, end support for Python < 3.6, add support
for Python 3.8 and 3.9, and make installation of the interactive pybdsf shell
optional (PR 169)
* Improve handling of the beam in the spectral index module (PR 165)
* Improve handling of large, complex islands (PR 160)
* Allow a file to be supplied for the ch0 image used in the spectral index module (PR 127)
* Plus fix many other minor bugs and issues

* Fix exception behaviour if spline order change does not work
* 2019/09/27 - Add check for frequency info in header

* Fix various shapelet decomposition issues
* Fix crash in Gaussian fitting (96)
* Fix blank_limit check_low error (100)
* Fix various minor bugs

* Add support for Python 3
* Fix various minor bugs

* Fix segfault in Gaussian fitting (63)
* Fix math domain error (76)
* Fix setup.py for boost versions > 1.63 (58)

* Fix an error on total flux density (50)
* Add the possibility to provide an external noise and mean maps (43)
* Append the image FITS header into catalog FITS header (53)
* Make PyBDSF compatible with newer boost libraries, specifically those used in Ubuntu 18.04 (55)