Add more tests/examples for lt.py

[utensil]

See discussions in #158.

See current coverage at https://codecov.io/gh/pygae/galgebra/src/master/galgebra/lt.py and the following examples that are not part of the tests yet:

• https://github.com/pygae/galgebra/blob/master/doc/python/LinearTrans.py
• https://github.com/pygae/galgebra/blob/master/doc/python/Ltrans.py
• https://github.com/pygae/galgebra/blob/master/doc/python/LtransInst.py
• https://github.com/pygae/galgebra/blob/master/doc/python/TensorDef.py

and they might overlap with https://github.com/pygae/galgebra/blob/master/test/test_lt.py .