Lower the minimum unit cell length when indexing

[vincefn]

Currently the unit cell starting length is 3.0 A, which is too high for a number of structures, e.g. Fe-alpha. In the COD, there are 430 (orthorombic) crystal structures with an 'a' unit cell parameter between 2.5 and 3, and 75 (including non-orthorombic) below 2.5A.

So the default value should be lowered, at least to 2.5, (see B and ZRNCl structures)