Dliswriter

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**API improvements & fixes**

* Removed representation code setters from ``Attribute``.
* More value type and representation code (repr code inferred from value) checks.
* Setting up ``Attribute`` 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.

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**Equivalent (parallel) frames**

* Made it easier to add frames with the same set of channel (dataset) names, but separate data - e.g.
two of each: DEPTH, RPM, and AMPLITUDE, coming from two separate measurements, associated with relevant frames
(FRAME_1, FRAME_2). See ``examples/create_dlis_equivalent_frames.py``.

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**Representation codes**

* Fixed representation codes for numpy dtypes.
* Tests & improvements for utils (repr code converter, source data wrappers, etc.).

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**Cleanup**

* Some name changes, restructuring.
* Updated README.

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**All DLIS objects added**

* Exposed all types of DLIS objects (WellReferencePoint, Group, Message, etc.)
in ``DLISFile`` through ``add_<object>`` methods (e.g. ``add_group``).

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**Instantiating ELFRObjects directly**

* Each ``EFLRObject`` (later renamed to ``EFLRItem``) can be initialised directly by calling the constructor
of the relevant class (before they were initialised through the corresponding ``EFLR``, later renamed to ``EFLRSet``).