v0.6.1-alpha

initial implementation of pvsyst_celltemp function (#552) (#628)

* initial implementation of pvsyst_celltemp function (#552)

* Reformat for stickler-ci

* Add error tests for pvsyst_celltemp

* Remove unused variables per stickler-ci

* Add pvsystem.pvsyst_celltemp function to docs

* correct api title

* formatting changes per wholmgren and cwhanse

* Additional formatting improvements

* Fix symbol in Reference