Feature/pskip multiple fields

[mgyoo86]

Extended the use of "pskip" (number of time steps between preconditioner updates) to multiple fields in time_step_split.f90:
s2_mat (Advancing Fields)
s8_mat (Advancing Density)
s9_mat (Advancing Pressure or Temperature)
s15_mat (Advancing runaway)

Avoiding frequent updates of preconditioners, which are very expensive,
the field solver's performance is greatly enhanced by a factor of 3 to 10.

The "pskip" value can be set in C1_input file, which is fixed during the simulation.
In the future, a more adaptive "pskip" determination could be implemented for better performance and stability of the simulation.

It is encouraged to use this pskip with the "initial_guess" feature to reduce the number of iterations in the field solver.

[bclyons12]

@mgyoo86 is this ready to merge?

@nferraro any concerns?

[mgyoo86]

@bclyons12
I believe this pull request has no conflicts with the master branch.
One minor thing that can be improved is to change the if statement
from "if(mod(ntime,pskip)==0)" to "if(mod(ntime-1,pskip)==0)".

This will ensure that the preconditioner is updated at the first time step (when ntime=1).
Otherwise, during the initial time of simulation (ntime<pskip), the un-preconditioned solver needs a lot of iterations to converge until when the preconditioner is updated at ntime==pskip.