Updated documentation to include estimator option.

As discussed in Issue #345 
Geez. Sorry. Kayve its time to move to an editor with spell check for this.
I will fix tonight.
On Dec 22, 2014 12:45 PM, "Paul Romano" <notifications@github.com> wrote:

> In docs/source/usersguide/input.rst
> <https://github.com/mit-crpg/openmc/pull/349#discussion-diff-22179426>:
>
> > @@ -1034,6 +1034,16 @@ The ``<tally>`` element accepts the following sub-elements:
> >
> >      *Default*: total
> >
> > +  :estimator:
> > +    The estimator element is used to force the us eof either ``analog`` or
> > +    ``tracklength`` tally estimation.  ''analog'' is generally less efficient
> > +    though it can be used with every score type.  ''tracklength'' is generally
> > +    the most efficient, though its usage is restricted to tallies which do not
>
> restricted to tallies *that* do not...
>
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> <https://github.com/mit-crpg/openmc/pull/349/files#r22179426>.
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