Micro-cache: memoize/cache function results

This is a lightweight mechanism to memoize results of functions that are often repeatedly called with the same arguments. It is indended for small-scale situations -- i.e., the function is called only with a small variety of arguments, for example less than 100, and the arguments (as well as results) are not too big (e.g., no large matrices or data sets). If too many different calls are to be cached, hlp_microcache "forgets about" the oldest ones. Its memory is also lost after MATLAB is closed or after typing "clear all". Different places of a program can independently memoize results of their functions, by calling hlp_microcache with their own unique 'domain' identifier. The overhead is in the ballpark of MATLAB's num2str(), i.e., very low.

Example:

% if this line is executed for the first time, it is as slow as magic(2000)
m = hlp_microcache('test',@magic,2000);

% if it is executed a second time, it is ca. three orders of magnitude faster than m=magic(2000)
m = hlp_microcache('test',@magic,2000);