Add opt-in option to template features

[mirpedrol]

Description of feature

Add opt-in option to template features. This is required for some new functionalities such as: #3272, #3314 and #3261

Needs:

• Turning off the switch for these features in the textual app by default
• Organising the switches into sections, with a specific section for opt-in features
• Adding these features to the template nf-core.yml