Adaptive volume_ratio setting

[singularitti]

As a reference, for $P(V)$ data, it is generally not advisable to let $\frac{ V }{ V_0 } < 0.8$, which corresponds to have volume_ratio should not be larger than 1.25 in qha. My opinion is that when fitting, if we find the volume_ratio tends to be larger than 1.25 in the first step, we could let qha use 5th order Birch--Murnghan eos automatically, and print a warnging.