Substrate absorption is not consistently accounted for

There are two parameters found in IFO models that refer to substrate absorption: ifo.Optics.SubstrateAbsorption and ifo.Optics.ITM.SubstrateAbsorption. It's not well documented which one is supposed to be used for what.

A quick grep suggests that ifo.Optics.ITM.SubstrateAbsorption is not currently used anywhere in the code, except for the obsolete gwinc function. ifo.Optics.SubstrateAbsorption is used only for a BS thermal lensing power limit check in ifo_power.

We should track total absorption in all substrates that are inside the PRC/SRC (ITMs and BS), in order to correctly estimate the power recycling gain and SRC loss. We should track absorption in each substrate for thermal lensing limits, and for heat budgeting purposes in cryo substrates.