More Index constructors

[mtfishman]

It may be nice to have more compact constructors for specifying the prime levels of an Index. Currently the interface is:

Index(2, plev=1)
Index(2, "x", plev=1)

We could add:

Index(2, 1)
Index(2, "x", 1)
Index(2, ("x", 1))

though the first one is unfortunate since it is pretty opaque when written like that.

Follow up to #686 (comment).

This is mostly nice for having more compact syntax for making sets of indices for the purpose of testing, such as:

N = 4
ds = (n + 1 for n in 1:N)
ps = (n - 1 for n in 1:N)
is = Index.(ds, ps)

Currently you would have to do:

is = [Index(d, plev=p) for (d, p) in zip(ds, ps)]

since broadcasting doesn't work through the keyword argument, so you can't do Index.(ds, plev=ps).

[mtfishman]

Also note that as of #686 you can do:

N = 4
ds = (n + 1 for n in 1:N)
ts = ("$n" for n in 1:N)
is = Index.(ds, ts)

which is pretty compact, but a similarly compact prime level version would be nice as well.