Add function to retrieve particular epigraphical markers

[goodmami]

This issue is spun off of #37. To find a particular type of marker for some triple, one must do something like:

for epi in g.epidata[triple]:
    if isinstance(epi, marker_type):
        # do something with epi

It would be convenient to be able to request those directly, e.g.:

g.get_marker(marker_type)

Questions:

• where does this function go? on the Graph class? in penman.epigraph?
• should there be separate functions to return (1) all markers of a type and (2) just the first one, or just one function? Aside from POP, there is generally never multiple markers of the same (leaf) type for a triple, so it seems more convenient to just get the first one.

[goodmami]

I'm adding a function specifically to get the variable that is pushed by a triple, if any, which resolves the original reason for this issue. Even though it doesn't solve it in the general case, perhaps these specific functions (like surface.alignments() and surface.role_alignments()) are more usable.