Optionally make use of 'parallel' in a package

[Daniel Kraft]

Hi!

In a package (level-set, to be precise) I want to make use of
'pararrayfun' of the parallel package to utilise parallel computation.
However, I do not want to make level-set strongly dependent on parallel.
 Instead, I would like to simply fall back to single threading when the
parallel package is not installed.

For this, I see two "problems" that I need to solve:

1) Decide whether or not a particular package (in this case, 'parallel')
is installed.  I think this is possible with both the 'pkg' and 'ver'
functions.  Is there some recommended way to do it?

2) If present, make sure that 'parallel' is loaded when I need it.  I
could use 'pkg load' / 'pkg unload' calls in PKG_ADD / PKG_DEL or the
called function itself.  However, this would mess with the user's
environment, right?  Is there a way to ensure that the package is loaded
only temporarily during my function without modifying the global
environment?

Thank you a lot!  Yours,
Daniel

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