Extract history to function

[truth-quark]

Tasks:

• Extract file history attribute to function
• [ ] Move set history to end of process() to group sman I/O operations (for future fixes)
• [ ] Move set CF conventions to end of process()

Unit tests can likely be skipped as the code is very small.

[truth-quark]

Small problem: moving the functions to update sman attributes changes the order in the NC file, which affects rough testing with diff (if a single byte changes, diff fails). Is there a way to bypass the iris output object sman & set the attributes? Are cube.attributes shared and thus global between all cubes? A hacky option is to set history on a cube & let iris save that to the NC file.

[truth-quark]

Moved sman & comments changes to issue #27. History extraction resolved by #43.

[truth-quark]

Also addresses a task in #14.