Allow instantaneous water heaters w/ larger tank volumes

[shorowit]

For example, this product is called an instantaneous water heater but it has a 34 gal tank:

• https://www.ahridirectory.org/Home/DownloadCertificate?ReferenceId=sUzbLVC3/aFB9wkj0dGAH/eKUzfZz8eK+bcRHuvG3FyFnilMp83//aEj1jp7zbZC
• https://www.americanwaterheater.com/residential/gas/pg10-34-150-2nv/

Presumably it's considered instantaneous because of the large burner rate.

We previously assumed that instantaneous water heaters have tank volumes <= 2 gal and thus insignificant standby losses. To handle products like this, we should allow tank volume as an input and, if it's greater than 2 gal, model it as a conventional storage water heater.

[shorowit]

We should also allow HeatingCapacity for instantaneous water heaters.

[shorowit]

And we should probably replace "tankless" with "instantaneous" in our code.