[feature] Bitfield record class

[ngjunsiang]

Introduction

The PDUs in IEEE1278.1-2012 contain structured, hierarchical records, of which three kinds can be identified:
A. fixed-size records, with a fixed marshalled size that is known upfront, and record fields have octet-multiple sizes
B. variable-size records, with a variable marshalled size that is not known upfront, and is typically included in one of the record fields as a length or count.
C. fixed-size bitfield enums/records, with fields that are non-octet-multiple sizes

This is a first attempt at implementing the last category (C), involving record fields with non-octet-multiple sizes.

All tests pass so far.

Changes

New method for reading bytes from stream

DataInputStream and DataOutputStream get read_bytes() and write_byte() methods respectively that allow reading an arbitrary number of raw bytes from the stream , and writing a byte sequence of arbitrary size to the stream.

New namespace, record

The new classes are in a separate file, record.py, to avoid dumping its required imports into dis7.

If this namespacing arrangement is preferable I will gradually migrate other records over to this module as I work on them as well. This will leave dis7.py primarily for PDU classes.

Sample bitfield class definitions

In record.py are a bitfield class factory function, bitfield(), and two sample classes, SpreadSpectrum and NetId. I have also pushed a small change to dis7.py, which affects ModulationType (which uses SpreadSpectrum) and thus the TransmitterPdu class as well.

Minor bugfix

Also included is a small fix to SignalPdu, left over from #56.

Requests

Since this PR proposes an interface for bitfields, I would prefer to merge it into a testing branch first and keep it apart from master until sufficient time has passed for users and contributors to give it a run.

If this is okay, please reject this PR and I will make another one for testing branch after it is created.

Related/affected issues/PRs

• The Bitfield class incorporates the proposal in Added marshalledSize to all classes #54 to add a marshalledSize() method that returns the bytesize of the complete record. Consider this an experiment for that proposal as well.
• The addition of bitfield records contributes to progress in null() Function Call? #14, which is held up by the absence of these record classes.
• bitfield classes, along with the marshalledSize() method, will make Boundary padding for records and PDUs #53 easier as it provides an easy way to obtain the length of records.
• The namespacing proposal above mirrors Confusing naming for identifiers #52 in its proposal to break up the dis7 module into multiple submodules so that it is easier to differentiate the classes in this package.
• While not currently open as an issue, the definition of SpreadSpectrum completes the implementation of ModulationType, which means that testTransmitterPdu.py can now incorporate the modulationType attribute into its testing as well (it currently stops at power)