Calculation of td_log in heat exchangers with constant temperature difference

[fwitte]

Discussed in #394

^{Originally posted by dk-teknologisk-enp February 24, 2023}
Currently it seems that if ttd_u == ttd_l for a heat exchanger, then td_log ends up as nan, and thus also the kA.

However, this is a perfectly valid situation, for example in a heat exchanger with purely boiling/condensing on both sides.

I suggest making the following changes to base.py, such that td_log is set to the constant temperature difference in case ttd_u == ttd_l:

(base.py:838-840)

[fwitte]

solved by #396