Remove all float-equality warnings in Matrices.cpp.

The three types of equality check (to -1, 0, and +1) are replaced by
inline functions with the float-comparison warnings turned off by a
gcc pragma.

It is still to be determined how to make this compatible with other
compilers.
Factor out isExactly* to a PrivateUtils file for reuse in BLASReplacement.cpp.

As of this check-in there are no more -Wfloat-equals warnings.
Merge branch 'float-warnings' of github.com:roryyorke/acado into roryyorke-float-warnings