Wtfix

**Enhancements**

- Add support for deleting a Field from a FieldMap by its tag name. E.g. `del message.PossDupFlag`.
- Add pre-commit hooks for checking code style and quality.
- Fix PEP8 code style violations reported by flake8.
- Update code quality dependencies (flake, black, etc.).

**Enhancements**

- Refactor `FieldSet` to better emulate the Python built-it container types.
- Rename `FieldSet` to `FieldMap`, `ListFieldSet` to `FieldList`, and `OrderedDictFieldSet` to `FieldDict`. Deprecate
and remove old classes.
- Implement all of the [`MutableSequence`](https://docs.python.org/3/library/collections.abc.htmlmodule-collections.abc)
abstract base class methods for `Field`.
- Operations can now be performed directly between `Field.value` and Python's built-in literals (e.g.
`Field(1, "abc") + "def"` will return `Field(1, "abcdef")`).
- Replace `as_str`, `as_bool`, and `as_int` with Python special methods to allow more natural casting using `str()`,
`bool()`, and `int()`. Add new `float()` method for casting `Field`s to float.
- Deprecate and remove `FieldValue` class.
- Field ordering is no longer significant when comparing `FieldMap`s with other `Sequence`s.
- Replace `raw` property for converting `Field`s and `FieldMap`s to a byte sequence with `bytes()`.
- Add `frombytes()` and `fields_frombytes()` methods to `Field` for creating new `Field` instances from byte sequences.
- Optimize memory usage of `Field`s by adding a `__slots__` attribute.
- Make `Field` instances hashable by implementing `__hash__` and `__eq__`.
- Add `__format__` implementation to `Field`, with a custom `t` option, for printing fields with their tag names.
- Convert the FIX representation of 'null' (`"-2147483648"`) to `None` when constructing a `Field` for more natural
usage in Python.
- Now Encodes boolean Field values to "Y/N".
- Remove `Message.get_group()` and `Message.set_group()` in favor of handling group fields like any other Field in the
message.
- Remove deprecated `InvalidGroup` exception.

**Enhancements**

- JSON encode / decode directly to and form FIXMessage instead of FieldSet.

**Enhancements**

- Split logging app into separate inbound and outbound processors so that the respective loggers can be injected in
different parts of the pipeline.
- Add JSON encoder / decoder for FieldSets - allows messages to be JSON encoded.

**Fixes**

- Fix WSGI callable to use ``session_name`` parameter correctly.

**Enhancements**

- Move `wsgi.py` to global `config` package.
- Add gunicorn support for running WTFIX and Flask in production environments.
- Add guidelines for doing production deployments.
- Provide ``JsonResultResponse`` structure for wrapping REST API responses.

**Fixes**

- Fix WSGI callable to use ``session_name`` parameter correctly.

**Enhancements**

- Add support for configuring multiple FIX sessions using the ``SESSIONS`` config parameter.
- Different pipeline connections can now be initiated by using the ``--sessions`` command line parameter with ``run_client.py``.
- New ``LoggingApp`` for logging of inbound and outbound messages.
- New ``RESTfulServiceApp`` for sending messages via a REST API.

**Fixes**
- Don't raise an exception if a heartbeat message (0) is received unexpectedly.