update joss paper DOIs (pvlib#544)

* add joss badge [skip ci]

* add holmgren2017 doi

* add doi [skip ci]

* add mikofski2017

* update pvlib doi [skip ci]

finalize whatsnew

finalize 0.5.1 whatsnew (pvlib#385)

set 0.5.0 release date

add release date to whatsnew

finish v0.4.4 whatsnew

update whatsnew for release

update pvsystem and tracking tutorials for pvlib 0.4 api changes (pvl…

…ib#281)

* update pvsystem.ipynb

* update tracking tutorial

* update whatsnew

set date in whatsnew 0.4.1

add release date to whats new