sdif time-oriented requests

[equilet]

From Olivier Pasquet on the Max forums:

• SDIF-tuples needs a way to access the data without knowing the
  dates. Something like "next" and "end".
• in this case, SDIF-tuples needs a second outlet with the non
  interpolated dates.
• SDIF-tuples needs a bang when the stream is finished.
• SDIF-info needs a way to dump the dates from the SDIF file.

[ramagottfried]

• re: next message -- I was actually thinking the same thing not long ago. The function is already in the object, but it's not exposed. I can take a look at that.

• re: outlet -- I'm not sure about needing another outlet -- the right outlet is there as the "info" outlet. we could easily send the time data out that outlet when the next message is sent.

• re: bang -- I don't think we need this, but yeah, that's a good idea to send an end of file message out of the info outlet when the next frame is off the end of the buffer.

• re: dump -- Actually there's an undocumented message "print" which prints all the frame times (with lots of other info) out the right outlet. we may want to reformat this a little but maybe best not to mess with legacy code too much, idk how many patches are relying on the print output format.

[ramagottfried]

note to self: we probably want to use SDIFbuf_GetNextMatrix which does a type check, in keeping with way that SDIF-Tuplet's GetMatrix function works -- this way you can get the next of a specific matrix type if you want.

[ramagottfried]

added next/prev messages and a "framebounds" message for when you get to the end. seems functional -- @equilet can you test it a bit?