Invalidate infinite low and high bin limits if not underflow/overflow bins

[GraemeWatt]

We plan to soon support the encoding of underflow bins (e.g. {low: -.inf, high: 0}) and overflow bins (e.g. {low: 250, high: .inf}). See HEPData/hepdata#358 and HEPData/hepdata-converter#55. The validator should be extended to ensure that infinite bin limits are not given elsewhere. For example, assuming only one independent variable, a value of -.inf should only be allowed for the first low bin limit and a value of .inf should only be allowed for the last high bin limit, but not elsewhere. This could be implemented by extending the check_independent_variable_values function. The case of multiple independent variables is more complicated to check.