Gempak pgrb2 spec should not modify DATA during execution

[WalterKolczynski-NOAA]

Description
The current JGFS_ATMOS_GEMPAK_PGRB2_SPEC is redefining $DATA between each ex-script call.

Requirements
$DATA should not be modified during execution.

Acceptance Criteria (Definition of Done)
Updated scripts that do not change $DATA without causing file collisions that could cause unexpected behavior.

Dependencies
Can likely be accomplished at the same time as #1223

[WalterKolczynski-NOAA]

Subset of #293