2187: fix export string for ec number fields

[bhutz]

This should fix the issue for #2187

[JohnCremona]

Thanks -- I am checking this now

[JohnCremona]

Good. I tried all three formats and they do load into the respective packages.

2 comments which are not intended to delay this bug-fix:

1. I don't know why we use [...] for the Magma lists; [...] would work.
2. I think it would be more helpful to users to actually create both the number field and the curves when loading the file.