Added numpy ndarray to TensorIndexType and TensExpr

In order to work with real data in a given basis, I added this PR to allow to add a numpy ndarray to a tensor expression or to a tensor index.

ndarrays on tensor indices are considered as the values of their metric tensors.
ndarrays on tensor expressions are considered as their values, in a given basis.

TODO list:
* [x] decide whether ndarrays should affect the `args` in `Basic`.
* [x] check that ndarray dimensions are compatible with `TensorIndexType` dimensions, both in metric and in tensor expressions.
* [x] add printer for `VTIDS`
* [x] add doctests to `VTIDS`
* [x] `canon_bp` currently does not rearrange tensor axes in ndarray.
* [x] add `applyfunc` doctests
* [x] add `strip` doctests
* [x] ndarray in `TensorIndexType` should be inherited by the metric.
* [x] add `VTIDS` link in .rst documentation files.
* [x] make sure all commented out tests are un-commented