Normalization by average_beta / local_beta

Proposal to fix the issue raised by @alphaover2pi over email.

Warning, this fix contains changes which will change the default behavior of core functions:

• Default behavior of Optics is changed when AT lattice is loaded: now default to optic function to s=0 instead of average ones.
• Default behavior of LongRangeResistiveWall and ImpedanceModel when an AT lattice is loaded is to normalized by average_beta / local_beta

To be discussed ...

Update:

• Adds Optics.average_beta attribute
• Both ImpedanceModel and LongRangeResistiveWall uses it.
• Add pytest markers for tests and update .gitlab-ci.yml and pyproject.toml files.
• Tags tests with markers.