Make LeBailFit/LeBailFunction work with Chebyshev.  Refs #7704.
Resolved conflict from merging master.  Refs #7704.
Fixed some error due to merging master.  Refs #7704.
Fixed the unit test due to previous change.  Refs #7704.
Fixed an issuer.  Refs #7704.

The issue was caused by re-definition of background function's order
'n'.  If there is no input vector of background parameters, the default
is set to n=0, A0 = 0.
Cleaned the codes.  Refs #7704.
Merge branch 'feature/7704_chebyshev_bkgd' into develop