* saving or loading a session from a file now includes `Axis` and `Group` objects in addition to arrays

(closes :issue:`578`):

Create a session containing axes, groups and arrays

>>> a, b = Axis("a=a0..a2"), Axis("b=b0..b2")

>>> a01 = a['a0,a1'] >> 'a01'

>>> arr1, arr2 = ndtest((a, b)), ndtest(a)

>>> s = Session([('a', a), ('b', b), ('a01', a01), ('arr1', arr1), ('arr2', arr2)])

Saving a session will save axes, groups and arrays

>>> s.save('session.h5')

Loading a session will load axes, groups and arrays

>>> s2 = s.load('session.h5')

>>> s2

Session(arr1, arr2, a, b, a01)

Note: all axes (groups) of a session are stored in the same CSV file/Excel sheet/HDF group

named __axes__ (__groups__).