Let Gaussian accept LinearOperator represented sqrtprec

[chaozg]

fixed #419

Issue: it might open some opportunities if we allow a Gaussian distribution to be initialized with scipy.sparse.linalg.LinearOperator.

What I did:

• update get_sqrtprec_from_sqrtcov in cuqi/distribution/_gaussian.py to accept aLinearOperator object as sqrtprec;
• add check_if_conditional_from_attr in cuqi/utilities/_utilities.py, following @nabriis 's comment below;
• check_if_conditional_from_attr is used in get_indirect_variables so that a Gaussian distribution initialized with LinearOperator is not detected as a conditional distribution (just a reminder: a LinearOperator is callable);
• add unit test test_Gaussian_from_linear_operator_sqrtprec in tests/test_distribution.py

Some notes (please correct me if I'm wrong here):

• For MCMC methods in general, we don't need logdet as it will be cancelled when evaluating the density function of the proposal and the current state;
• In CUQIpy, we need logdet defined to compute logpdf: I think this is somehow unnecessary and the unnormalized version _logupdf should be enough in place of logpdf. But I have to admit that I'm not confident if this point still holds when it comes to Gibbs sampling. (This issue turned out to be an echo from the old days #16, as Nicolai pointed in the comments below)
• In order to make a Gaussian distribution initialized with LinearOperator work with the current samplers in CUQIpy except ROTs, we need to set logdet to some value. It doesn't matter to what value it is set as it will be cancelled.
• For RTOs, we don't evaluate logpdf, so we don't need to set logdet.

[chaozg]

Thanks @nabriis for your review and suggestions. I've updated the code to address your comments and am now requesting your further review.

[chaozg]

Thanks @nabriis , I'm now requesting @jakobsj 's review.