The grid spacing is reduced locally around each atom, and the reduction is
given by 1/(1+A), where A is specified by this variable. So, if
A=1/2 (the default), the grid spacing is reduced to two thirds = 1/(1+1/2).
[This is the $A_{\alpha}$ variable in Eq. 2 of F. Gygi and G. Galli, Phys.
Rev. B52, R2229 (1995)]. It must be larger than zero.
It must not be larger than two, as the coordinate transformation is not bijective
anymore for larger values of A. This caps the local refinement that can be
obtained at 1/(1+2), i.e. a factor of three, which is the governing limitation when
attempting all-electron calculations on heavy atoms.