Fixed bug in the LongLinkTerm class. Previously, this class contained
const double/float LongLinkTerm::naik_coeff&. Although a constant reference
can be used to prolong the life of a temporary in C++, this does not hold when the
constant reference is part of a class. This resulted in an error in the single-precision
long-link force computation where the lifetime of the temporary generated by static_cast<float>(coeff)
was not extended.
Merge branch 'hisq_force_fix' into quda-0.7