tankmix2: Stagnant zone volume incorrectly zeroed when there's no net flow

[endearqb]

Issue #1: Stagnant zone volume incorrectly zeroed when there's no net flow

Current Behavior:
When there is no net flow (inflow = outflow), vt = 0, and mixzone->v = vmz, the code incorrectly sets stagzone->v = 0 even when the stagnant zone contains water (stagzone->v > 0).

Expected Behavior:
When there is no net flow, the stagnant zone volume should remain unchanged if no water is being transferred between zones (vt = 0).

Code Location:
In tankmix2() function, the else branch of volume update section:

else
{
    mixzone->v += vnet;
    mixzone->v = MIN(mixzone->v, vmz);
    mixzone->v = MAX(0.0, mixzone->v);
    stagzone->v = 0.0;  // This line incorrectly zeros the stagnant zone
}

---

Issue #2: Mixing zone volume calculation incorrect during partial stagnant zone emptying

Current Behavior:
When water is transferred from the stagnant zone to mixing zone but net outflow is larger (vt > 0 && vt < -vnet), the code sets mixzone->v = vmz, maintaining maximum volume despite net water loss.

Expected Behavior:
The mixing zone volume should account for both the water received from stagnant zone and net outflow:
mixzone->v = vmz + vt + vnet

Code Location:
In tankmix2() function:

if (vt > 0.0)
{
    mixzone->v = vmz;  // Should consider both transfer volume and net flow
    if (vnet > 0.0) stagzone->v += vt;
    else            stagzone->v = MAX(0.0, ((stagzone->v) - vt));
}

Proposed Fix:

if (vt > 0.0)
{
    if (vnet > 0.0)
    {   
        stagzone->v += vt;
        mixzone->v = vmz;
    }
    else
    {
        stagzone->v = MAX(0.0, ((stagzone->v) - vt));
        mixzone->v = vmz + vt + vnet; // fix the issue 2
    }
}
else
{
    mixzone->v += vnet;
    mixzone->v = MIN(mixzone->v, vmz);
    mixzone->v = MAX(0.0, mixzone->v);
    if (vmz - mixzone->v >0.0) stagzone->v = 0.0; // fix the issue 1
}

[LRossman]

The fix has been applied in PR #821 .