Fix non-initialized mixing ratio in p/z-lev interp (#2104)

TYPE: bug fix

KEYWORDS: mixing ratio, interpolation, pressure level, height level, missing value

SOURCE: Jesus Fernandez, Andrés Simón-Moral and Josipa Milovac (Instituto de Física de Cantabria, CSIC-Universidad de Cantabria, Spain)

DESCRIPTION OF CHANGES:
Problem: The mixing ratio field is not being initialized in the interpolation routine. This leads to no missing values below ground and values from previous time steps filling these areas.

Solution:
The mixing ratio has been initialized to missing values, as for the rest of variables.

LIST OF MODIFIED FILES:
```
M       phys/module_diag_pld.F
M       phys/module_diag_zld.F
```

TESTS CONDUCTED: 
The Jenkins tests are all passing.

RELEASE NOTE: Fix non-initialized 3D moisture field in pressure- and height-level diagnostic interpolation.

phys/module_diag_pld.F

@@ -82,6 +82,7 @@ SUBROUTINE pld ( u,v,w,t,qv,zp,zb,pp,pb,p,pw,                    &
                ght_pl(i,kp,j) = missing
                s_pl  (i,kp,j) = missing
                td_pl (i,kp,j) = missing
+               q_pl  (i,kp,j) = missing
             END DO
          END DO
       END DO

phys/module_diag_zld.F

@@ -82,6 +82,7 @@ SUBROUTINE zld ( u,v,w,t,qv,zp,zb,pp,pb,p,pw,                    &
                ght_zl(i,kz,j) = missing
                s_zl  (i,kz,j) = missing
                td_zl (i,kz,j) = missing
+               q_zl  (i,kz,j) = missing
                p_zl  (i,kz,j) = missing
             END DO
          END DO