API improvements and fixes

[the-mysh]

• Removed representation code setters from Attribute.
• More value type and representation code (repr code inferred from value) checks.
• Setting up Attributes' values and units together using dict or a new AttrSetup class.
• DLISFile: add_origin method; origin instance or setup keywords no longer accepted in DLISFile init.
• Support for defining multiple dlis files within one session (script).
• Removed logging formatting from library root.
• Exposed most frequently needed objects for import from library root (from dlis_writer import ...).
• Explicit init arguments and docstrings for most frequently used classes.
• Saving date-time in GMT rather than 'local time'.
• Passing file header and storage unit label initialisation arguments directly to DLISFile.
• General refactoring & typing fixes.