Make `model.sequence` member optional

Note that the matching/selecting routines will just fail with some
undefined errors, if they are used anyway.

Theoretically, this might now work with some plain cpymad models. In
practice, there are still issues (expecting 'ex','ey' set in beam,
no automatic tuning of MultipleLocator, etc)
Add entrypoint for LHC models

This is only temporary, until a locator + entry point is added to
cpymad.