Addresses #135 and #137: when an undefined variant or axes is used in the experiments matrix, an error is thrown
adds new Config._axes field (currently an OrderedDict with the value being the key itself)
adds two simple test experiment setup files with an undefined variant and an undefined axes (identical to examples/sorting/sorting_cpp except for minor typos)
Addresses #135 and #137: when an undefined variant or axes is used in the experiments matrix, an error is thrown
Config._axesfield (currently anOrderedDictwith the value being the key itself)examples/sorting/sorting_cppexcept for minor typos)