Harmonise domain and system interface

[opeltre]

Complex now inherits from domain so that K.field, K.zeros, K.randn... is transfered but both interfaces are not fully consistent.

For starters:

• Replace calls to K[key] by K.get(key) to retrieve cells as we only want K[degree] to extract graded component.
• See what K.__iter__() should do for system/domain instances: yield degrees K[d] / self=K[0] ? remove it?

[opeltre]

1) K.field etc. solved by #11 2) K[key] calls are almost all replaced by K.get(key) - rest is quite linked to #14

Much functionality could be passed to the Fiber/Cell type, for instance when assuming keys are simplicial. This could be passed with something like

K = Sheaf({k: shape[k]}, ftype='simplicial', graded=0)

def Sheaf__init__(...):
    self.ftype = Fiber if ftype == None else SimplicialFiber

where SimplicialFiber has extra functionality from Chain e.g. Chain.read(key) -> maybe just call it Simplex

[opeltre]

The Complex type should just inherit from a Union [Sheaf] type for the access to graded components (simplicial structure on fiber keys might be done by fiber_type=Simplex argument)

Harmonise to K.get(key, degree=0)?

[opeltre]

solved by #17