add "Delay Material"

[JamesPHoughton]

From vensim docs:

The same as DELAY FIXED except that delay time can be a variable. If delay time is decreasing, some values of the input will be added to more recent inputs for output. If delay time is increasing, missval will be used when no output is available at a time. This function is particularly useful for modeling queues and production processes with varying and often random processing times.

probably wants to be part of an overhaul of the delay object, to also address #147

[ZakkaUgihRizqi]

Well, this is a very important function since most delay time is variable, but still not available. Hopefully, it will be available soon.

[enekomartinmartinez]

@mak63-dev we can comment here any issue related to the implementation of DELAY MATERIAL