allow user to clear have_fcn caching of optional functionality

[rdzman]

The have_fcn() function detects the availability of optional functionality (e.g. 3rd party solvers) and caches the result, including version and release date info if available. This cached information can become invalid if the Matlab/Octave path is subsequently modified, leading to the potential for incorrect behavior based on the now invalid cached results returned by have_fcn().

Currently this cache can be cleared for a given functionality by toggling it off then back on. E.g. for ipopt ...

have_fcn('ipopt', 0);
have_fcn('ipopt', 1);

It would be useful to be able to easily clear all of the cached information to force a recheck for all optional functionality, e.g. after modifying the Matlab path.

Something like ...

have_fcn('all', 'clear_cache');

We could also include the following as a shortcut for the two steps of disabling and re-enabling a particular functionality mentioned above.

have_fcn('ipopt', 'clear_cache');

[rdzman]

I should also mention that calling mpver is a good way to cause MATPOWER to check for (and cache) the availability of many of the optional solvers/functionalities at one go.