Isotope constructor now calls Isotope::initializeThermalScattering

the default values are hard-coded in the isotope constructor for
now. Default values may need to be changed later and/or let
users to change?
rescale isotope's xs during Material::addIsotope(..)

By default even log bin in energy is used, and the start and
end energy is the default ones set in Material class. May need
to update.
Implemented tally routine in Isotope::collideNeutron

 - Added a few more collisionTypes in Isotope.h to match the
 reaction rates that tally class can support;
 - an ugly switch statement is implemented for now; this method
 is not very efficient, may need to change later;
 - nothing is being donw for the leakage rate for now because
 I am not sure what I should do.
Implemented tally routine in Material::collideNeutron

pretty much copy & paste the implementation in
Isotope::collideNeutron. Things may need to be updated later (see
previous commit).