fix: what fluid to recirculate when there is no rate entering a compressor stage in a multiple streams compressor train at a given time step

[olelod]

This regards VairableSpeedCompressorTrainCommonShaftMultipleStreamsAndPressures only. In situations where there is both an additional ingoing stream and an additional ingoing stream between two compressors, one can end up with a situation where one tries to mix together two streams with zero mass rate (to mix the one stream passing from compressor 1 to compressor 2 and the additional stream entering the compressor train between compressor 1 and compressor 2). This means that if we look at this time step isolated there is an unknown composition to recirculate in the second compressor. This fluid to recirculate should have the same composition as the fluid passing through the second compressor at the previous time step.

What does this pull request change?

• Includes a class attribute keeping track of the composition of the fluid passing through the different compressor stages in a compressor train, such that it will be available for the next time step (except for the first time step - there the code will fail)
• A small fix giving the correct standard condition density for the resulting fluid with mixing two fluids (now using the correct pressure and temperature, instead of assuming that the given pressure and temperature is the standard condition pressure and standard condition temperature)
• Adding a test