String representation

[gubaidulinvadim]

Work in progress!
Modified pyaml_repr to print nested string representation. For BPM, this gives better results.

sr = Accelerator.load("tests/config/EBSOrbit.yaml")
bpm_design = sr.design.get_bpm("BPM_C04-04")
print(bpm_design)
    BPM(description=None, lattice_names='BPM_C04-04', name='BPM_C04-04', peer=Simulator(name='design', lattice='/home/gubaidulin/codes/operation/pyAML/pyaml/tests/config/sr/lattices/ebs.mat', mat_key=None, linker=None, description=None))

There are a few issues still that can be resolved in different ways.

• There's no x_pos/y_pos (control system name string) in the string representation. It was present before.
  • We could make x_pos and y_pos 'public' by removing '_'. Then they would appear automatically in the string representation.
  • Another option is to make an 'include' keyword, similar to 'exclude' and explicitly include some attributes.
• I'm worried if nested string representation will not have the same issues as we had before with very long and unreadable string representation. Maybe there should be a length limit?

When solved resolves #345