LNM Variables: allow omitting start value

[mhier]

Needs to be done after #187: Allow to omit the tag for variables. In that case, no initial value will be pushed to wait_for_new_data accessors, until a value has been written to the variable (usually through another accessor).

To be clarified: What to do when a poll-typed read is done before a value has been written? Throw a runtime_error? Blocking is not allowed by the specification... @killenb Any ideas?