Allow symbolic vectors to be simplified elementwise

[jvkersch]

This patch adds a class Vector_symbolic_dense to sage.modules, whose functionality at this stage consists solely of providing a simplify_full method. The need for such functionality came up in the review of #10132 (metric surfaces in 3D), where symbolic vectors with complicated entries show up, and explicitly calling simplify_full all the time would be cumbersome (see also #10552).

The modifications implemented by this patch are the following:

• Adds the class Vector_symbolic_dense;
• Adds functionality to the element_class function in sage.modules.free_module to use this class;
• Changes some doctests in free_module_element.py (because vector([1, 2, pi]) is now a symbolic vector rather than an element of a generic free module).

Eventually, this class could be expanded along the lines of sage.matrix.matrix_symbolic_dense.

[robertwb]

Short and sweet. Looks good to me. More methods added at #11381.

[jvkersch]

Hi Robert, thanks for the quick review!

[jdemeyer]

The commit message of the patch should not be one very long line. Please wrap it into multiple lines but try to make sure that the first line makes sense by itself (this is what hg log shows).

[jvkersch]

symbolic vector class

[jvkersch]

Hi Jeroen, thanks for the notification. I've updated the patch accordingly.

[jdemeyer]

Joris, in the future it would be better to have the ticket number in the filename, for two reasons:

1. it improves greppability.
2. it ensures that no two patches from different tickets have the same filename.