PerformanceProfiler: be nicer when classes' dependencies are missing

In that case, the code could never be executed anyway, so it is
irrelevant when reporting the performance statistics.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Put PerformanceProfiler under BSD

This is the same license as ImageJ2, where this code will eventually
be moved anyway.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add Javadocs to the PerformanceProfiler

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
PerformanceProfiler: reduce performance impact when not profiling

We do not need to obtain the start time in every method when not
profiling anyway...

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