Fixes #258

This PR implements the Vensim DEPRECIATE STRAIGHTLINE function as required in #258.

The C implementation is similar to FIXED DELAY. It uses a ring buffer to accumulate contributions from an input stream uniformly spread over a depreciation time. Unlike FIXED DELAY, which is a special kind of level, DEPRECIATE STRAIGHTLINE is an aux variable. The implementation assigns a variable subtype of depreciation to trigger special handling in code gen. Also unlike FIXED DELAY, it is necessary to initialize the ring buffer values to zero, because the buffer contents will start to be emitted on the very first time step. (The FIXED DELAY buffer has to fill up first before emitting any data.)

A new depreciate test model demonstrates the function.

The fisc fiscal period argument is not supported in this implementation.