Common pipeline functions in narps_open.core

[bclenet]

Changes proposed in this Pull Request:

• 🐍 Adding a narps_open.core.common module with general purpose functions to be used inside pipelines (common to nearly all pipelines)
• 🐍 Adding a narps_open.core.image module with functions related to image manipulation to be used inside pipelines
• 🔬 tests added accordingly

Checklist:

• Descriptive title
• Targets the main branch
• Changes are functional
• My code is explicit and comments were added to it
• Code conforms with PEP8
• Tests were added for the changes and they complete successfully
• Existing tests were updated (if needed) and they complete successfully
• Documentation was updated