[FEA] Add cache manipulation routines

[cliffburdick]

Currently the cache in MatX is stored in a static variable that has no way for the user to control it. Items are added transparently to the cache as needed, and they are never removed until the application exits. This can cause problems for applications that grow and use the cache more than smaller applications. At a minimum the user should be able to delete the cache of a specific operation, a specific operator, or the entire cache.

Something like the following would be suitable, but the syntax is only for an example:

1. delete_cache(a = matmul(e,f) * fft(b)) or delete_cache(fft(b))
2. delete_cache(FFT1D_OP); // Deletes all 1D FFT cache
3. delete_cache(); // delete entire cache

In the first example multiple caches may be deleted if multiple functions are embedded in a single operation, whereas the second part only disables the one FFT operation.