Add validation rules for SED-ML L1V4

[jonrkarr]

From #57

For L1v4, the additional restrictions apply:

• Style
  • Network of base styles should be acyclic (Style.base)
  • Color should be a 6 or 8 digit case-insensitive hexidecimal string (/[a-f0-9]{6,6}|[a-f0-9]{8,8}/i)
  • LineStyle.thickness should be non-negative
  • MarkerStyle.size should be non-negative
  • MarkerStyle.lineThickness should be non-negative
• Parameter estimation
  • A ParameterEstimationTask must have at least one AdjustableParameter, and at least one FitExperiment.
  • A FitExperiment must have at least one FitMapping.