CH4 streams file has to use a fixed date or results in roundoff exact restart problems

### Brief summary of bug

Rather than using the current date to advance the stream data for the CH4 finundation stream a fixed date is being used because when the date varies exact restarts can cause roundoff differences. This is even though the streams file has a single time sample and uses

### General bug information

**CTSM version you are using:** clm4_5_16_r242

**Does this bug cause significantly incorrect results in the model's science?** No

**Configurations affected:** BGC simulations

### Details of bug

Here's the FORTRAN code where use of the current date is commented out and a set date is used.

``` fortran
! Explicitly set current date to a hardcoded constant value. Otherwise
! using the real date can cause roundoff differences that are
! detrected as issues with exact restart. EBK M05/20/2017
! call get_curr_date(year, mon, day, sec)
year = 1996
mon = 12
day = 31
sec = 0
mcdate = year*10000 + mon*100 + day
```