Implementaion of Container and mixed loaders (H4EP001)

With hickle 4.0.0 the code for dumping and loading dedicated objects
like scalar values or numpy arrays was moved to dedicated loader
modules. This first step of disentangling hickle core machinery from
object specific included all objects and structures which were mappable
to h5py.Dataset objects.

This commit provides an implementaition of hickle extension proposal
H4EP001 (telegraphic#135). In this
proposal the extension of the loader concept introduced by hickle 4.0.0
towards generic PyContainer based and mixed loaders specified.

In addition to the proposed extension this proposed implementation inludes
the following extensions hickle 4.0.0 and H4EP001

H4EP001:
========
    PyContainer Interface includes a filter method which allows loaders
    when data is loaded to adjust, suppress, or insert addtional data subitems
    of h5py.Group objects. In order to acomplish the temorary modification
    of h5py.Group and h5py.Dataset object when file is opened in read
    only mode the H5NodeFilterProxy class is provided. This class will
    store all temporary modifications while the original h5py.Group
    and h5py.Dataset object stay unchanged

hickle 4.0.0 / 4.0.1:
=====================
    Strings and arrays of bytes are stored as Python bytearrays and not as
    variable sized stirngs and bytes. The benefit is that hdf5 filters
    and hdf5.compression filters can be applied to Python bytearrays.
    The down is that data is stored as bytes of int8 datatype.
    This change affects native Python string scalars as well as numpy
    arrays containing strings.

    numpy.masked array is now stored as h5py.Group containin a dedicated
    dataset for data and mask each.

    scipy.sparce matrices now are stored as h5py.Group with containing
    the datasets data, indices, indptr and shape

    dictionary keys are now used as names for h5py.Dataset and
    h5py.Group objects.

    Only string, bytes, int, float, complex, bool and NonType keys are
    converted to name strings, for all other keys a key-value-pair group
    is created containg the key and value as its subitems.

    string and bytes keys which contain slashes are converted into key
    value pairs instead of converting slashes to backslashes.
    Distinction between hickle 4.0.0 string and byte keys with converted
    slashes is made by enclosing sting value within double quotes
    instead of single qoutes as donw by Python repr function or !r or %r
    string format specifiers. Consequently on load all string keys which
    are enclosed in single quotes will be subjected to slash conversion
    while any others will be used as ar.

    h5py.Group and h5py.Dataset objects the 'base_type' rerfers to 'pickle'
    are on load automatically get assigned object as their py_object_type.
    The related 'type' attribute is ignored. h5py.Dataset objects which do
    not expose a 'base_type' attribute are assumed to contain pickle string
    and thus get implicitly assigned 'pickle' base type. Thus on dump for all
    h5py.Dataset objects which contain pickle strings 'base_type' and 'type'
    attributes are ommited as their values are 'pickle' and object respective.

Other stuff:
============
    Full separation between hickle core and loaders

    Distinct unit tests for individual loaders and hickle core

    Cleanup of not any more required functions and classes

    Simplification of recursion on dump and load through self contained
    loader interface.

    is capbable to load hickle 4.0.x files which do not yet
    support PyContainer concept beyond list, tuple, dict and set
    includes extended test of loading hickel 4.0.x files

    contains fix for labda py_obj_type issue on numpy arrays with
    single non list/tuple object content. Python 3.8 refuses to
    unpickle lambda function string. Was observerd during finalizing
    pullrequest. Fixes are only activated when 4.0.x file is to be
    loaded

    Exceptoin thrown by load now includes exception triggering it
    including stacktrace for better localization of error in debuggin
    and error reporting.

hickle/loaders/load_builtins.py

@@ -260,7 +260,6 @@ def load_none_dataset(h_node,base_type,py_obj_type):
 def load_list_dataset(h_node,base_type,py_obj_type):
     """
     loads any kind of list like dataset
-
     Args:
         h_node (h5py.Dataset): the hdf5 node to load data from
         base_type (bytes): bytes string denoting base_type 

hickle/tests/test_hickle.py

@@ -193,7 +193,6 @@ def test_65bit_int(test_file_name):
     j_hkl = load(test_file_name)
     assert j == j_hkl
 
-
 def test_list(test_file_name):
     """ Dumping and loading a list """
     filename, mode = 'test_list.h5', 'w'