Declare Sections with Name only

[Michael Jung]

At this stage we have the following difference between sections and tensor fields:

sage: M = Manifold(2, 'M', start_index=1)                  
....: X.<x,y> = M.chart()                            
....: E = M.vector_bundle(2, 'E')      
....: e = E.local_frame('e')      
....: v = M.vector_field('v')      
....: s = E.section('s')
Traceback (most recent call last)
...
IndexError: string index out of range

This simply comes from the fact that the method section does not like pure strings as input, in contrast to vector_field or tensor.

[Michael Jung]

New commits:

​490d38d

Trac #30228: init comp with string

[git]

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

​78692a6

Trac #30228: misleading comment corrected

[Travis Scrimshaw]

LGTM.