Add reversible to_str meta-property about linkml

[cmungall]

Adding some notes to clarify the use case for string_serialization as a slot property

biolink/biolinkml#301 has serialization/parsing for the canonical serialization for a class, which is great, but it can also be useful at the slot level

Consider

classes:
  person:
     slot_usage:
       firstname:
       lastname:
       fullname:
         description: "E.g. Chris Mungall"
         string_serialization: "{firstname} {lastname}"
       phonebook_entry:
         description: "E.g. Mungall, Chris"
         string_serialization: "{lastname}, {firstname}"
       journal_entry:
         description: "E.g. Mungall, C"
         string_serialization: "{lastname}, {firstname}[0:1]"
    string_template: "{firstname} {lastname}"

(the journal_entry be out of bounds for expressivity but just adding for illustrative purposes)

The class level string_template may be redundant with the canonical fullname but I think that is OK

The main point is that it's useful to have different serializations for a slot

In all cases, the interpolation is evaluated at the level of the containing class instance

This is also useful for representing dosdps, e.g. a blml class may have seperate serializations for rdfs:label, owl:equivalentClass, ...