Cork lr

[meleg]

In this PR the low-rank data structure for the CORK pencil is implemented.

Here is the checklist: (transferred from #227)

• matrix-types has to be decoupled in CORK_pencil
• better names to XXLinearization-types (or restructure it)
• verification (plot the eigenvalue error as function of number of terms in the expansion)
• implement low-rank COR_pencil_structure
• documentation
• tests
• remove / lessen duplication of code: nleigs_coefficients.jl and method_nleigs.jl
• remove the file nep_transformations.jl (check that all was transferred to NEPTransformations.jl)

[jarlebring]

I suggest we try to wrap up all CORK-related things in this PR. So I moved the checklist.

[jarlebring]

All checkboxes checked except two which can be done in a separate PR.