rationale:
  - we accept that Frida will fail for array sizes > 2^15
  - we cannot exclude that complete conversion to size_t would
    cause underflow errors
warning -Wno-sign-compare
replace gsl_sort_index by new function triv::sorted_indices
initialize a few variables to prevent warning

result: no warnings from -pedantic, almost none from -Wall

declaring most indices as size_t
this will be reverted immediately

existing citations