Allow to get elements by (unique made) names from line.get_table()

[eltos]

Description

Unique names from line.get_table() (and survey and ...) cause an error if trying to get the corresponding element via line[name].

While for line.get_table() there is env_names to help out, for survey there is nothing. Also, this is quite unintuitive behaviour.

line = xt.Line(
        elements = {"obm": xt.Bend(length=0.5)},
        element_names= ["obm","obm"]
    )
print(line.get_table())
line["obm::0"]

Table: 3 rows, 8 cols
name                   s element_type isthick isreplica parent_name iscollective env_name  
obm::0                 0 Bend            True     False None               False obm       
obm::1               0.5 Bend            True     False None               False obm       
_end_point             1                False     False None               False _end_point

KeyError: 'Name obm::0 not found'

The PR fixes this. See xsuite/xplt#31 (comment)

Closes xsuite/xplt#31

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts

[giadarol]

Thank you!