“missing” value indicator semantics about octave-tablicious

[apjanke]

I think I'm making progress on this: using eqn, which is a wrapper for x == y | (isnanny(x) & isnanny(y)), we can get to generic missing-indicator testing, which is enough to implement the global/generic ismissing, and rmmissing flows from that.

standardizemissing then just needs dynamic detection of type-specific standard missing values.

I've added global ismissing() and rmmissing(), with indicator support in ismissing().

0e4aae3
9cc3199

I think we can probably include a mostly-full implementation of “missing” functions in this package.

[apjanke]

Done with all of this except for a couple of the modes in fillmissing(). Calling this finished.