Pipe methods can return complex valued ouputs

[MassimoCimmino]

Pipe class methods can return complex-valued outputs. This was observed when running unit tests in pipes_tests for the MultipleUTube configuration.

This could be fixed using np.real() either on the Multipole method (preferably) when computing the thermal resistance matrix or in the class methods themselves.

[MassimoCimmino]

It seems the cause is the calculation of eigenvalues in MultipleUTube._update_model_variables(). The coefficient matrix _A calculated from the resistances is correctly real-valued.

https://github.com/MassimoCimmino/pygfunction/blob/7d37b8b3bf4b349701681e42281c21cccb54ca42/pygfunction/pipes.py#L1551-L1552

The two options are:

• Take the real value of the eigenvalues/eigenvectors using np.real(). The imaginary part should be negligible.
• Calculate the matrix exponentials using scipy.linalg.expm(). This eliminates the need for eigenvalues but may affect the calculation time.