adding macros for zero matrix and vector

They can be used to initialize vector- or matrix-like parameters
to zero in the constraints.
add zero array initializer
adding helper function which returns zero for a given type
convert 0 to proper data type

Some beta function might be identical zero (0).  However, the
corresponding parameter is not necessarily a scalar.  So, we have to
convert the scalar 0 to the data type of the parameter.
adding sarah/ directory to stroe our own models

I also added an extended MSSM model file with vector-like parameters
(for testing only).