fix bugs, add reconstructors, update FBPUNetReconstructor, add reference reconstructors module, add utility datasets, refine reconstructor param saving and loading interface (docs), update docs

[jleuschn]

• fix bugs
• add CT reconstructors: dip, tvadam, learnedgd, learnedpd, iradonmap
• update FBPUNetReconstructor
• add reference_reconstructors module from which pretrained reconstructors can be loaded by get_reference_reconstructor, which currently provides the low-dose CT reconstructors from https://github.com/oterobaguer/dip-ct-benchmark
• add utility dataset classes
• refine reconstructor param saving and loading interface (docs), enabling to save or load all parameters by only specifying path to save_params or load_params
• update docs