Improve speed and precision of exc and nsec functions using uniroot.all

[beckyfisher]

ecx and nsec values are currently estimated using an x vector based on x_range and precision, with x estimated as the x value of this vector closes to the target y value. This would be more elegantly handled using uniroot - however noting that uniroot.all would be required in the case of an hormesis model, with the second solution (if there are more than one) most likely corresponding to the desired estimate.