Add Flex100 to the list of available detectors

[jmunozv]

Flex100 corresponds to the theoretical NextFlex detector configured to emulate the actual Next100 detector.
This MR adds Flex100 to the list of detectors that can be analyzed with IC.

[jmbenlloch]

Everything looks good but you forgot to commit the file invisible_cities/database/localdb.Flex100DB.sqlite3 itself. That copy of the database commited too. It has to be uploaded via LFS so everyone can get that file when checking out this branch or cloning the repo.

[jmunozv]

Everything looks good but you forgot to commit the file invisible_cities/database/localdb.Flex100DB.sqlite3 itself. That copy of the database commited too. It has to be uploaded via LFS so everyone can get that file when checking out this branch or cloning the repo.

Done.

[jmbenlloch]

This PR includes the necessary changes to add a new detector: Flex100.