Fix variable extraction with intervals

[lfoppiano]

Variables with intervals are not parsed correctly:

Superconductivity has been observed in all samples with x ⩾ 0.05 and the maximum critical temperature (T c ) ≈ 32 K has been obtained in samples with 0.1 ⩽ x ⩽ 0.2 from electronic resistivity measurement.

results in the extraction of 0.1 ⩽ x ⩽ 0.2 linked to 32K, however the material 0.1 ⩽ x ⩽ 0.2 is wrongly parsed as x =0.1

[lfoppiano]

Another example, different but related: x is more than 0.05:

<variable>x</variable> is <value>more than 0.05</value>

Result:

variable="x"
value= "m, e than 0.05"

[lfoppiano]

From @kensei-te

As a user, what is appreciated is that, we substitute and produce only for endpoints. For instance if you have "La3-xSix x=0-0.1, Tc is 10 K for all", then "La2.9Si0.1 Tc 10 K" and "La3 Tc 10 K".