Add number concentration calculations to YOG code.

[jatkinson1000]

This PR should close #18 by adding a calculation for number concentration tendency to the YOG code using the updated cloud liquid and ice tendencies.

• The number concentration calculation is taken from the clubb routines.
• Number concentration tendency from YOG is allowed to be negative, but cannot reduce total number concentration below zero.
• Number concentration tendency from YOG is added as an optional output to the statistics files.

[paogorman]

This code looks good.
Regarding the check that number concentrations not go negative: It may not be necessary because I just noticed that there is already a check for number concentrations to stay non-negative in physics_update:
if (m /= ixnumice .and. m /= ixnumliq .and. &
m /= ixnumrain .and. m /= ixnumsnow ) then
call qneg3(trim(ptend%name), state%lchnk, ncol, state%psetcols, pver, m, m, qmin(m:m), state%q(:,1:pver,m:m))
else
do k = ptend%top_level, ptend%bot_level
! checks for number concentration
state%q(:ncol,k,m) = max(1.e-12_r8,state%q(:ncol,k,m))
state%q(:ncol,k,m) = min(1.e10_r8,state%q(:ncol,k,m))
end do
end if
On the other hand, we have these kind of checks throughout our yog scheme so maybe it is consistent to have it also.

[MarionBWeinzierl]

Looks good to me, only one minor comment