heat exchangers in simscape part 1: LMTD

LMTD approach to calculate temperatures and heat transfer. Based on pipe blocks.

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