starting to add a new dump_type: "strain_only"

this is pointwise strain on a subset of the gll points, with depth and distance filtering.
Useful for instaseis with spatial downsampling
actually dumping the strain now in strain_only mode

some remaining problems with the fluid
adding strain_only mode to fieldtransform
write rmin etc as well for strain_only mode and

defaults to full planet in case of fullfields mode
fixing a bug in fluid strain output
output axis s=0 also in the fluid
(re)introducing jbeg and jend for kernel wavefields in

fullfields or strain_only mode. Useful for instaseis to get denser lateral then depth
sampling