Fixes for WDM6 and WDM7 (#1933)

Fixes for WDM6 and WDM7 (#1933)

TYPE: bug fixes

KEYWORDS: WDM schemes, uninitialized value, wrong cloud autoconversion rate

SOURCE: Songyou Hong, PSL/NOAA, internal

DESCRIPTION OF CHANGES:
Problem:
1. cloud autoconversion rate is off by a factor of 600
2. if cloud water is present, cloud number concentration needs to be set at initialization.

Solution:
Both problems are fixed in this PR.

LIST OF MODIFIED FILES:
M    phys/module_mp_wdm6.F
M    phys/module_mp_wdm7.F

TESTS CONDUCTED:
1. Tested in case studies.
2. The Jenkins tests are all passing.

RELEASE NOTE: This PR fixes two errors in the WDM6 and WDM7 that have been in the code since V4.0: 1. uninitialized cloud number concentration when cloud is present at the start. 2. The cloud autoconversion rate was off by a factor of 600. The effect of the first error isn't large, but the effect of the wrong autoconversion rate has resulted in doubling the surface rainfall in some tests (warm rain processes).