Madseq

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- fix check for filename extension

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- fix some sliced multipole coefficients
- handle QUADRUPOLE.K1S in makethin
- fix a naming problem when slicing with template
- fix python3 problems

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- allow .json files to be passed as the -s parameter (extension is used to
distinguish .json from .yaml)

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- use different names for slices
- fix sequence name in comment before templates

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- add 'use_at' slicing option that enables to use AT values

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- internal: remove attribute access for Element properties
- fix bugged Element.copy and .__contains__ regarding to base elements
- fix JSON/YAML serialization errors
- fix bugged implementation for predefined elements
- fix bugged LOOP style
- fix bugged makethin
- fix bugged use_optics
- rename 'use_optics' => 'template'. technically, this is a change in the
public interface, but we are still in 0.X and furthermore the feature was
not working before anyway.