New features: Functional Graphical Lasso and lambda1 mask

[fabian-sp]

New features

• Functional Graphical Lasso solver example and utilities
• SGL problem allows lambda1_mask in reg_params

Improvements

• Tests of glasso_problem are all seeded and with more assertions

Other

• some cleanup of experiment_utils functions
• changes in README

[fabian-sp]

To-Do for this branch:

• remove decorator hard-coded version in the requirements, the bug has been fixed by now (https://stackoverflow.com/questions/66922359/unexpected-error-while-drawing-networkx-graph)
• change to np.random.seed(None) for the synthetic data generator

[fabian-sp]

This PR also

• removes pinned dependencies (decorator, sphinx and jinja)
• requirements needed for docs only are specified in a separate file