Utility functions to acquire components in a system

[ijpulidos]

In order to avoid having redundant code for the different protocols, there are a few utility functions that seem useful to have ready to be used.

• One that get the alchemical components
• One that get components of specified type
• One that gets ALL components in a system

Whether these should live here or in gufe can be discussed further.

[IAlibay]

One that get the alchemical components

Currently that exists in openfe under system_validation. However @ianmkenney recently added a diff method in gufe that effectively does this without any extra work.

One that gets components of a specified type

Would definitely want to see this in gufe.

all the components in a system

That's just ChemicalSystem.values()?