[Dev] Add examples

[ocots]

Add:

• MRI
• Kepler

[ocots]

See also Betts book, the second edition.

[ocots]

See this document from PSOPT: PSOPT Application Examples Document. The examples are here: https://github.com/PSOPT/psopt/tree/master/examples. Actually, we could update the existing problems in OptimalControlProblems that are part of PSOPT.

[ocots]

See this:

• COPS
• PSOPT
• MINTOC
• DYMOS
• BOCOP
• BETTS

[jbcaillau]

also: add a swimmer

@PierreMartinon has a nice example in CTDirect (see also first sprint with SED - sweet memories)
https://github.com/control-toolbox/CTDirect.jl/blob/main/test/problems/swimmer.jl
https://github.com/control-toolbox/CTDirect.jl/blob/main/test/problems/swimmer2.jl (scalarised version)

[ocots]

Consolidated into #257 — all problem sources (ICLOCS, COPS, PSOPT, MINTOC, DYMOS, BOCOP, Betts) and specific problems (MRI, Kepler, swimmer) have been extracted and added there. Closing this issue.