Added feature for processing nested structures

[msfur]

Enables processing for nested structures, as mentioned in and fixes #308 and on Stackoverflow. Takes into account nested structures in the following form:

    substructure_def = (
         ("iVar", pyads.PLCTYPE_INT, 1),
         ("sVar", pyads.PLCTYPE_STRING, 1)
     )

    structure_def = (
         ("iVar1", pyads.PLCTYPE_INT, 1),
         ("structVar", substructure_def, 2)
     )

or

    structure_def = (
        ("iVar1", pyads.PLCTYPE_INT, 1),
        ("structVar",
            (
                ("iVar", pyads.PLCTYPE_INT, 1),
                ("sVar", pyads.PLCTYPE_STRING, 1)
            ), 2
        )
    )

Contains adaptations to size_of_structure(), dict_from_bytes() and bytes_from_dict() and the associated tests.

[coveralls]

Pull Request Test Coverage Report for Build 8985140072

Details

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 94.432%

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Totals
Change from base Build 8519762609:	0.03%
Covered Lines:	1713
Relevant Lines:	1814

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💛 - Coveralls

[chrisbeardy]

@msfur this is awesome, looks great as a solution for the nested structs issue and thanks for adding the tests in too! If possible, could you please add a documentation example at the end of this section too? https://pyads.readthedocs.io/en/latest/documentation/connection.html#structures-with-multiple-datatypes

I will then merge.

[msfur]

@chrisbeardy I have made the desired change to the documentation and added an example.

[keenanjohnson]

This looks great! Any way I can support getting this merged?