Change from an implicit assumption of editable data streams

[eshadatta]

When reviewing #60, Scot brought up metadatum.rb and mentioned that instead of having an assumption of immutability implicit by saying that a datastream could not be edited if it's a source datastream, change it so that there's a constant that states if a datastream is read only or editable. Also, there was discussion about the word immutability and it was decided that it would be changed to editable or read only.