TensorFreeModule._an_element_: Create a default basis in the base module if necessary

[mkoeppe]

Likewise for ExtPowerFreeModule etc.

The current code creates an element without components if the base module has no default basis. Such elements are not useful for running the test suite because adding them gives a "no common basis" error.

That currently all tests work depends in a subtle way on the order in which modules are created.

In this ticket, we propose to change the _an_element_ method to create a default basis in the base module if none already exists -- this matches what FiniteRankFreeModule._an_element_ already does.

This will simplify further refactoring of the code, such as the identification of modules discussed in #30241.

[git]

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

​d2d7f42

ExtPowerDualFreeModule._an_element_: Make sure a default basis is available

[git]

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

​9e739e5	FreeModuleLinearGroup._an_element_: Make sure a default basis is available
​eec19fd	TensorFreeModule._an_element_: Make sure a default basis is available

[egourgoulhon]

Thanks for fixing this!

[mkoeppe]

Thanks!