Shestakov Kershaw distorted mesh

This function creates a highly distorted quadrilateral mesh, known as a Shestakov mesh.

This routine generates an nm by nm mesh, commonly known as a
Shestakov mesh, with 0 < r < rm and 0 < z < zm. The randomness
is controlled by parameter "a", where 0 <= a <= 0.5. A value of
a = 0.5 gives a rectangular mesh. Boomerang zones are allowed
but bowties are not allowed.

Based on the original Fortran program by Alek Shestakov; that program
is found in: Nuclear Science & Engineering, Vol 105, pp.88-104 (1990),
"Test Problems in Radiative Transfer Calculations", by A. Shestakov,
D. Kershaw, and G. Zimmerman