fix process_csv

[ptpt]

Fixes #399

[naho2196]

I have an issue using column_indexes = [col - 1 for col in column_indexes] instead of column_indexes = map(lambda x: x - 1 if x else None, column_indexes). The error is the following:

File "c:\python39\lib\site-packages\mapillary_tools\process_csv.py", line 270, in
column_indexes = [col - 1 for col in column_indexes]
TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'

Probably somebody know the reason and how to fix it? thanks

[ptpt]

Sorry @naho2196 I didn't noticed the case that the column indexes could be None. It's fixed in #406