Add func=None param to functions in cache.py

[NimaSarajpoor]

@seanlaw suggested that it might be a good idea to add func=None to functions in stumpy/cache.py such as cache._enable() and cache._clear(). Adding func could allow enabling/clearing of a single function.

[seanlaw]

It is one thing to add the parameter but one needs to check and ensure that the functions are actually being "enabled" for caching and properly "cleared" (and eventually "recompiled" when the cache._recompile() function is added.

It's possible that this may not be worth adding.