Checklist

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• Please read Contributing to QuantumToolbox.jl.
• Any code changes were done in a way that does not break public API.
• Appropriate tests were added and tested locally by running: make test.
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Description

Summary of this PR:

New structures:

• AbstractSpace
• Space
• AbstractDimensions
• Dimensions
• GeneralDimensions

New field

• dimensions (originally called dims) for Qobj / QobjEvo / TimeEvolutionProblem / EigsolveResult

New property

• dims for Qobj / QobjEvo / TimeEvolutionProblem / EigsolveResult

New internal functions:

• get_dimensions_to
• get_dimensions_from
• dimensions_to_dims: transform AbstractDimensions to integer-vector
• _get_dims_string: to get the dims string for printing

Changed internal functions:

• _check_dims -> _gen_dimensions. This was the one to check the format of user specified dims, now it also checks the format and generates the Dimensions structure.
• check_dims -> check_dimensions. This was the one to check if the two Qobj have the same dims, now it allows multiple input Qobj and check whether all Qobj.dimensions are equal).

Related issues or PRs

close #271
close #353