(L1 products) `grid_mapping` Reference Missing in Metadata Cubes

[nemo794]

Description: Metadata Cubes datasets declare a grid_mapping attribute pointing to "projection", but no corresponding dataset named "projection" exists in the group.

Affected Products: L1 products only (RSLC, RIFG, RUNW, ROFF)

CF Requirement: Section 5.6 states grid_mapping attributes must reference an existing coordinate variable that contains projection information.

Occurrences by Product:

RSLC (9 instances):

File: NISAR_L1_PR_RSLC_001_149_D_080_2005_QPDH_A_20251003T130952_20251003T131016_X05016_N_P_J_001.h5

Variables with grid_mapping="projection" attribute:
  - /science/LSAR/RSLC/metadata/geolocationGrid/coordinateX
  - /science/LSAR/RSLC/metadata/geolocationGrid/coordinateY
  - /science/LSAR/RSLC/metadata/geolocationGrid/heightAboveEllipsoid
  - /science/LSAR/RSLC/metadata/geolocationGrid/alongTrackUnitVectorX
  - /science/LSAR/RSLC/metadata/geolocationGrid/alongTrackUnitVectorY
  - /science/LSAR/RSLC/metadata/geolocationGrid/elevationAngle
  - /science/LSAR/RSLC/metadata/geolocationGrid/groundTrackVelocity
  - /science/LSAR/RSLC/metadata/geolocationGrid/incidenceAngle
  - /science/LSAR/RSLC/metadata/geolocationGrid/losUnitVectorX

Problem: Variable "/science/LSAR/RSLC/metadata/geolocationGrid/projection" does NOT exist
Fix: Either create this variable with CRS metadata OR remove grid_mapping attribute from all above datasets

RIFG, RUNW, ROFF (9 instances each):

Same pattern as RSLC:
Variables under /science/LSAR/{PRODUCT}/metadata/geolocationGrid/* reference grid_mapping="projection"
But variable "/science/LSAR/{PRODUCT}/metadata/geolocationGrid/projection" does NOT exist

Fix: Create the projection variable or remove grid_mapping references

cc: @hfattahi @gshiroma @bhawkins @xhuang-jpl @seongsujeong