Added new protokernel and new datatype

Added new protokernel: volk_gnsssdr_8i_max_s8i.h that returns the max
value in an array. Not finished yet.
Added new file gnss_sdr_complex.h which defines a new datatype:
gnss_sdr_complexc: complex numbers with 8-bits integer real part and
8-bits integer imaginary part. Complex integer numbers are not defined
in ISO C99, but some compilers (clang, GCC in lasts versions) support
them. This could be problematic and should be reviewed and taken into
account.