Reserved namespace policy for future extension of SeqCol

[ahwagner]

One comment that came up during the Connect meeting that I thought was really interesting was the idea of conventions for a reserved array namespace in SeqCol. The idea being that if future properties are identified that would become part of the specification, it would provide a mechanism for adding those properties without potentially colliding with implementation-defined array names. An idea proposed was that a case-sensitive convention could be used for this purpose, which may have implications for #33.

[nsheff]

We made a decision on this, which is outlined in this ADR:

#49