heat exchangers in simscape part 1: LMTD

For steady state considerations, you can often use the LMTD approach to calculate temperatures and heat transfer for heat exchangers.
Instead of writing a new simscape block, I used the standard block from thermal_liquid, overrode the internal heat transfer and calculated the LMTD.
for use:
- heat exchanger dimensioning & quick'n'dirty check in overall plant

please note:
- not working for flow reversal and highly dynamic simulation

potential for optimization:
- heat transfer coefficient from lookup-table
- maybe replacing LMTD with NTU approach