omp critical directives

[RobWatersMet]

Simialr to #3198

Ideally we'd like to replace exisiting omp directives with identical equivalents from a transformer - This will help ensure the directives are not mistakenly stripped out in the future and will improve consistency in how omp directives are being applied to UKCA.

E.g.:

! OMP CRITICAL will only allow one thread through this code at a time,
! while the other threads are held until completion.
!$OMP CRITICAL (asad_bedriv_init)
IF (first_pass) THEN
  IF (first) THEN
    first = .FALSE.
    ! find N2O5, NO3, and NO2 species in species array
    DO js = 1,jpspec
      SELECT CASE (speci(js))
      CASE ('N2O5      ')
        in2o5 = js
      CASE ('NO3       ')
        ino3  = js
      CASE ('NO2       ')
        ino2  = js
      END SELECT
    END DO

    ! find N2O5 + M and NO2 + NO3 + M in termolecular reactions array
    DO js = 1,jptk
      SELECT CASE (spt(js,1))
      CASE ('N2O5      ')
        tn2o5   = ntrkx(js)
      CASE ('NO2       ')
        IF (spt(js,2) == 'NO3       ') tno2no3 = ntrkx(js)
      CASE ('NO3       ')
        IF (spt(js,2) == 'NO2       ') tno2no3 = ntrkx(js)
      END SELECT
    END DO