Handle 2 stitcher instances in same Windows directory

[ahelsing]

If 2 stitcher instances run in the same directory on Windows, the 2nd process gets this error:

 C:\Users\lnevers\Downloads>stitcher deletesliver lnwin7
 Traceback (most recent call last):
   File "stitcher.py", line 466, in <module>
   File "stitcher.py", line 447, in main
   File "stitcher.py", line 307, in call
 WindowsError: [Error 32] The process cannot access the file because it is being used by another process

Apparently on Windows, the os.rename of the existing stitcher.log to a new copy fails, where on Linux the first copy becomes stitcher.log.1 but the process continues to write to it successfully.

[ahelsing]

Options I see:

1. This new copy of stitcher writes to something named other than stitcher.log
2. Exit with a suggestion of running in a different directory or using --fileDir

[ahelsing]

Tom suggests that stitcher should behave as many other programs do: if you download a file in your browser and the suggested filename is taken, it adds a number to the end and renames the new file.

Additionally, this renaming would handle the rename problem (this issue). And it would address Niky's issue #789 to have the stitcher log files named from when the run was.

Logs could be named something like stitcher-YYYYMMDDHH24MI.log.

Documentation would have to be updated to help experimenters find the appropriate log file.

[ahelsing]

Doing the simple thing for now; a nicer error message. Full solution pushed off to a later release.