improve instructions on lattice import

[moellep]

When you import a lattice, the code is assuming the original code based on the file extension. So if you import a file "foo_madx.in" into madx, you might expect it to import via madx, but instead *.in maps to OPAL lattice import.

The text is currently: "Select Command (.ele), Lattice (.lte, .in, .madx, .seq), or Sirepo Export (.zip)"

But should be more explicit, something like:

Select a lattice for import:
 (*.madx, *.seq) MAD-X
 (*.ele, *.lte) elegant
 (*.in) OPAL
 (*.zip) Sirepo export file