Support missing sections

[ganctom]

If there is a missing section in the original data and consequently in the parsed folder structure, this leads to error during loading sections for alignment, because currently it is not handled.

In the following example, the original stack does not have a section #2, which fails to be interpreted by sofima_tasks.py at line 102

/tungstenfs/scratch/gmicro_share/_software/CondaEnvs/Linux/faim_sofima/lib/python3.9/site-packages/sbem/tile_stitching/sofima_tasks.py", line 102, in
block.sections[(i, grid_index)] for i in range(start_section, end_section + 1)
KeyError: (2, 0)

[ganctom]

@TomasGancarcik to verify in current em-alignment pipeline version.