Compacting the display of tensor product

[Dicolevrai]

With this ticket, through the methods :meth:`~sage.tensor.modules.free_module_tensor.FreeModuleTensor.display` and :meth:`~sage.manifolds.differentiable.tensorfield.TensorField.display`, SageMath is now able to compact the display of tensor product in the form dx^2. In the case the frame element is something like f^2, to avoid any misunderstanding in f^2^2, one can set compact_product=False in display to get the old displaying style f^2*f^2.

Raising a tensor field to a given power is also possible now. When the tensor is an automorphism field the composition law of maps is used instead of tensor product and in this case, raising to a nonpositive power is possible.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​b5412cc	nothing
​885e0a9	Compact tensor product display

[chapoton]

red branch => needs work

[mkoeppe]

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.