Change the way "fill" handles edge missing values.

[timcera]

Currently, at least for "--method=linear", handles edge values by linear interpolation to the time-series mean set for the time-stamp immediately before (if leading missing values) or after (if ending missing values). Should make an option, perhaps "edge"?

Most methods wouldn't need this...

--method	Missing values at beginning	Missing values at end
ffill		X
bfill	X
2.3	X	X
linear	uses time-series mean	uses time-series mean
index	uses time-series mean	uses time-series mean
values	uses time-series mean	uses time-series mean
zero	?	?
slinear	?	?
quadratic	?	?
cubic	?	?
spline order=n	?	?
polynomial order=n	?	?
barycentric	?	?
mean	X	X
median	X	X
max	X	X
min	X	X
from	X	X
krogh	?	?
piecewise_polynomial from_derivatives	?	?
pchip	?	?
akima	?	?