Load PHD2 star image from memory instead of temporary file

Summary: Currently Ekos saves the JSON formatted star image to temporary FITS file and loads it back, but CFitsio also supports simple memory file system that can be used instead saving writes to disk. Also reduces log spam as there is a debug print about each FITS load if debug logging is active.

Test Plan: Use PHD2 as usual and observe star images still being shown.

Reviewers: lancaster, mutlaqja

Reviewed By: lancaster, mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D25376