Sequence parser

[szymonlopaciuk]

Description

Provide features required by xsuite/xtrack#515:

• Add a BaseRef._formatted(formatter) method that returns a string corresponding to a representation of a Ref/and Expr according to the formatter.
• Add a _set_to_expr method that can reset the expression calculating the value of said ref.
• Introduce a LiteralExpr to enable the possibility of wrapping scalars: this can allow us to preserve the tree of an expression in full.
• Add a Manager.structure field that maps a ref to the refs that structurally depend on it (array element to the array, attribute to the owner, etc.). This is needed for fast retrieval of expressions from their "parent" containers.

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts