Option indexexchange in order to shorten notation in mcphas.j

The indexexchange parameter discussed in section 7.1.4 may be useful in keeping the input of the exchange components short. For example the $\zeta$ and $\eta$ strain modes on the quasi-cubic sites of a double hexagonal close packed structure involves the coupling of $P_{zx}$ ($I_g$) and $P_{xy}$ ($I_d$) as well as $P_{yz}$ ($I_e$) and $O_2^2$ $(I_h)$ quadrupoles, so the diagonalexchange=1 option cannot be used. Thus 64 components need to be specified to describe the $8\times 8$ exchange tensor. However, we can simplify this with the indexexchange parameter by setting diagonalexchange=2, as follows for UPd$_3$:

# UPd3 
#<!--mcphase.mcphas.j-->
#! a=9.92465 b=5.73 c=9.65  alpha=  90 beta=  90 gamma=  90
#! r1x= 1 r2x=   0 r3x=   0
#! r1y= 0 r2y=   1 r3y=   0
#! r1z= 0 r2z=   0 r3z=   1
#! nofatoms=1 nofcomponents=8
#*************************************************************************
#! x=   0 [a] y=   0 [b] z=   0 nofneighbours=6 diagonalexchange=2 gJ=0.8 sipffilename=mcphas.cf1
#! x[a]  y[b]    z[c]   Ja[meV]  Jb[meV]  Jc[meV]  Jd[meV]  Je[meV]  Jf[meV]  Jg[meV]  Jh[meV]
#! symmetricexchange=1 indexexchange= JaJa JbJb JcJc JdJd JeJe JfJf JgJg JhJh JgJd JhJe
 0    0    0.5 -0.022 -0.022 -0.022  0.0035198 -0.18643  -0.00053332 -0.18643   0.00087996  0.013      0.0064378
 0    0   -0.5 -0.022 -0.022 -0.022  0.0035198 -0.18643  -0.00053332 -0.18643   0.00087996  0.013      0.0064378
 0.5  0.5  0   -0.022 -0.022 -0.022 -0.0034195 -0.019714 -0.00034044 -0.019714 -0.00085486 -0.0065221 -0.0050197
-0.5 -0.5  0   -0.022 -0.022 -0.022 -0.0034195 -0.019714 -0.00034044 -0.019714 -0.00085486 -0.0065221 -0.0050197
 0.5 -0.5  0   -0.022 -0.022 -0.022 -0.0034195 -0.019714 -0.00034044 -0.019714 -0.00085486 -0.0065221 -0.0050197
-0.5  0.5  0   -0.022 -0.022 -0.022 -0.0034195 -0.019714 -0.00034044 -0.019714 -0.00085486 -0.0065221 -0.0050197