Defines ISmearcepter which is the interface a smearcepter must
implement. Enforces the implementation of a single function
which takes a FitEvent pointer and returns a pointer to a newly
allocated RecoInfo instance. A ISmearcepter subclass is welcome
to return a RecoInfo pointer to a RecoInfo subclass if extra
reconstruction information is needed. It is up to the interpreting
code to downcast this to a known subclass and clear up.

Smearcepters are configured by blocks in an input xml config file.
Instances of known and configured smearcepters are managed by the
Smearcepterton singleton.

Currently the only ISmearcepter subclass implemented is a simple
threshold accepter, aptly named ThresholdAccepter.

While the interface is intended to be abstract and resuseable,
the only implemented way to use and analyse smearcepted events
is through the Smearceptance_Tester measurement which is implemented
as an 'output format' in nuisflat. See the usage text of nuisflat
for details on command line arguments used.