Allow the ImgOpener to switch off file grouping

In this developers' performance tests, reading a single .tif from a
directory with 10,000 other files had a huge performance impact. It
seems that the TiffReader tries to read the whole directory every
time a slice is read, even if only one .tif file is read in the end.

This is a bit over the top, so let's add an option to switch that off,
keeping it on by default.

My gut feeling is that the default is wrong: most users will not care
about grouping files, and will not even know that ImgOpener tries to do
that! So let's discuss (between users, not between software architects
who do not use their own software on a daily basis) whether the default
should be changed, and if so, commit it as its own commit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ImgOpener: switch off file grouping by default

It is almost never what developers want, and it is quite surprising,
too, when developers wonder why reading files from a well-populated
directory is dramatically slower.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Let ImgOpener call SCIFIO's setAllowOpen(false)

SCIFIO has this option that it can prevent readers from opening a
stream just to determine whether they handle this type.

Of course, it would be better to just reuse the memory-mapped stream
because most readers need to inspect only the first couple of bytes
anyway.

This speeds up Tobias' benchmark from 317ms to 186ms.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ignore .factorypath files

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Clean up .gitignore

There is no need to add **/ rules.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>