Review current practices of PtyScan.

[bjoernenders]

This parent issue should collect all issues regarding changes and improvements to the PtyScan class.

• How should positions be loaded? The role of load_positions #3
• Verify if the problems in MPI data preparation #51 are still present
• Creating a .ptyd file from scratch #114
• PtydScan needs a review (info vs meta) #115
• auto_center not being read from .ptyd? #118
• Clean up info/meta/common and subclass PtydScan #124
• no consistency in using single/double precision #395
• Reconstruct portion of a scan #116

[bjoernenders]

Here is a comment from Pierre on an earlier issue which I changed into a discussion thread due to its open nature. See #447

The default pipeline should be simplified for straightforward cases while keeping more advanced features. For this, one could rehabilitate the DataSource class, which currently is not useful. Consider the possibility of having Ptypy act as a daemon, with data reading provided by completely separate processes.