remove MirrorSparseMatrix; delete enclone_visual (#521)

Removes everything that depended on the known-dead MirrorSparseMatrix type removed in 10XGenomics/enclone_ranger#384

Deletes enclone_visual and everything I could find relating to it.

Sets tests to panic instead of existing the process.

.gitignore

@@ -2,8 +2,6 @@ target
 enclone_exec/test/inputs/version14/85679/outs/raw_feature_bc_matrix/matrix.bin
 enclone_exec/testx/outputs
 enclone-data
-enclone_visual/outputs
-enclone_visual/unofficial_images
 stamps/data_version_in_use
 stamps/last_sweep
 stamps/last_linktest