Minor cleanup.

Rename some variables, remove a "raw" array (use a std::vector instead).
Try not to use raw arrays allocated with "new".

Also: remove silly pointer arithmetic magic in columnSystemCtx.
PIO::compute_start_and_count() supports t_count > 1.
Remove some uses of "const char *" and "const char []".

Remove a kludge in IceFlowLaw::IceFlowLaw(). Fix a bug (the enhancement
factor "e" was initialized only when the option prefix was set; my
fault).

This was prompted by #285 and
https://github.com/pism/pism/issues/285#issuecomment-50067444 (code
review requested by @bueler).
Use one-sided differences to compute w and strain heating at all margins.

Before this commit we used centered differences in the strain heating
computation and one-sided differences to compute vertical velocity, but
only at ice shelf margins.