Bug fix in UDM MP

[Songyou184]

The first line should be a single-line "purpose" for this change

TYPE: Bug fix

KEYWORDS: Prevent artificial rain accumulation

SOURCE: songyou hong

DESCRIPTION OF CHANGES:
Problem:
UDM MP occasionally produces unphysical rain activities

Solution:
Allow tiny values of rain drops (qr < 1.e-9) to be sedimented

LIST OF MODIFIED FILES: module_mp_udm.F

TESTS CONDUCTED:

1. Ming Chen confirmed that this PR fixes the issue in 4.8 UDM MP
2. Are the Jenkins tests all passing?

RELEASE NOTE:

[weiwangncar]

The regression results:

Test Type              | Expected  | Received |  Failed
= = = = = = = = = = = = = = = = = = = = = = = =  = = = =
Number of Tests        : 23           24
Number of Builds       : 60           57
Number of Simulations  : 158           150        0
Number of Comparisons  : 95           86        0

Failed Simulations are: 
None
Which comparisons are not bit-for-bit: 
None

[dudhia]

This should be targeted for develop.
The change in WDM7 includes the change for UDM, so it should go in after this one.

[weiwangncar]

This needs to be merged before PR-2366.