(InSAR) Some attributes are variable-length strings; should be fixed-length strings

[nemo794]

As agreed to by NISAR ADT, all datasets and their attributes should be fixed-length strings (not variable-length strings). In brief, this is to optimize HDF5 performance.

Attributes which are currently written as variable-length strings should instead be written as fixed-length strings:

Impacted Products: RIFG, RUNW, GUNW, ROFF, GOFF:
science/LSAR/GUNW/metadata/processingInformation/parameters/**reference**/frequencyA/slantRangeSpacing -> description
science/LSAR/GUNW/metadata/processingInformation/parameters/**secondary**/frequencyA/slantRangeSpacing -> description

Impacted Products: GUNW, GOFF:
science/LSAR/GUNW/grids/frequencyA/pixelOffsets/[x|y]Coordinates -> standard_name
science/LSAR/GUNW/metadata/radarGrid/[x|y]Coordinates -> standard_name

Impacted Product: GUNW:
science/LSAR/GUNW/grids/frequencyA/pixelOffsets/HH/[x|y]Coordinates -> standard_name

science/LSAR/GUNW/grids/frequencyA/unwrappedInterferogram/HH/[x|y]Coordinates -> standard_name
science/LSAR/GUNW/grids/frequencyA/unwrappedInterferogram/[x|y]Coordinates -> standard_name

science/LSAR/GUNW/grids/frequencyA/wrappedInterferogram/HH/[x|y]Coordinates -> standard_name
science/LSAR/GUNW/grids/frequencyA/wrappedInterferogram/[x|y]Coordinates -> standard_name

Impacted Product: GOFF:
/ -> title (root of product)
science/LSAR/GOFF/grids/frequencyA/pixelOffsets/HH/layer[1|2|3]/[x|y]Coordinates -> standard_name

Datasets checked:
NISAR_L1_PR_RIFG_013_118_A_171_014_2000_SH_20260222T102514_20260222T102520_20260306T102515_20260306T102521_X05016_P_P_J_001.h5
NISAR_L1_PR_RUNW_007_106_A_140_008_7700_SH_20251211T140849_20251211T140926_20251223T140850_20251223T140926_X05016_N_P_J_001.h5
NISAR_L2_PR_GUNW_001_062_A_019_006_2000_SH_20250927T114508_20250927T114542_20251126T114516_20251126T114545_X05016_P_F_J_001.h5
NISAR_L1_PR_ROFF_007_106_A_140_008_7700_SH_20251211T140849_20251211T140926_20251223T140850_20251223T140926_X05016_N_P_J_001.h5
NISAR_L2_PR_GOFF_005_106_A_140_006_7700_SH_20251117T140848_20251117T140925_20251129T140849_20251129T140925_X05016_P_P_J_001.h5

cc: @hfattahi @xhuang-jpl @oberonia78 @seongsujeong