Fix some error in thermal neutron TOF peak profile and add wiki page

[wdzhou]

The calculation on the Lorentzian part in the peak profile ThermalNeutronBk2BkExpConvPV. Complete the implementation.

Modify the definition of Sig-0, Sig-1 and Sig-2 to make it conformed to the definition in manual from what are used in Fullprof .irf file. In Fullprof's .irf file,
Sig0 = sig0^2^; Sig1 = sig1^2^; Sig2 = sig2^2^, where sig0, sig1 and sig2 are defined in manual, such that
sigma^2^ = sig0^2^ + sig1^2^ d^2^ + sig2^2^ d^4^.

Add wiki page for this function.

[wdzhou]

This issue was originally trac ticket 5779