rs.read_precognition(): Update docstring and set spacegroup after parsing log

[JBGreisman]

It came up today that rs.read_precognition() reads .ii files. This isn't explicitly stated in the docstring. It should be made more clear that the io function supports both file types.

[dennisbrookner]

Related to this issue, there's a bug in the logfile parsing of rs.read_precognition() where the spacegroup is parsed from the log file, but never actually assigned to the DataSet. I'll fix both of these in the same PR

[dennisbrookner]

Following additional discussion offline, this PR will

• Clarify in docs that .hkl and .ii files are allowed
• Fix bug in which space group is not parsed from log files
• Clarify in docs that user-supplied space group and/or cell will take precedence over those found in log file
• Add a warning for when user supplies both a space group and/or cell and a log file

[JBGreisman]

Awesome -- thanks! Two small clarifications:

1. I think the spacegroup is parsed correctly, it is just not set as a DataSet attribute.
2. I think the warning should only be raised if the user supplies all 3 arguments (sg, cell, and log). I could see a valid use to providing just a spacegroup or a cell if you only want to manually overwrite one attribute. I think the docstring should specify the precedence, but a warning should only be raised when the logfile parsing will not be used.