feat: implement DEPRECIATE STRAIGHTLINE

[ToddFincannonEI]

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.

[chrispcampbell]

@ToddFincannonEI: This all looks like a clean addition to me, thanks.

Re: this comment in the PR description:

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

I think it would be good to have a table in the README (or somewhere in the docs) that lists out all the supported Vensim functions along with any per-function limitations/caveats. I just filed #265 today to track improvements I've been wanting to make to the README and docs in general, so assuming I add that table of supported functions as part of that work, I can include a note about fiscal period being unimplemented at this time.