[WIP] Custom Encoder/Decoder for YAML Format

[EZoni]

This is a prototype of custom encoding/decoding for YAML format that could be done to achieve what was discussed in #3 (review) and #3 (comment).

The custom encoding/decoding produces this (custom implementation)

kind: Line
line:
- Drift:
    length: 0.25
    name: drift1
- Quadrupole:
    MagneticMultipoleP:
      Bn1: 1.0
    length: 1.0
    name: quad1
- Drift:
    length: 0.5
    name: drift2
- Quadrupole:
    MagneticMultipoleP:
      Bn1: -1.0
    length: 1.0
    name: quad2
- Drift:
    length: 0.5
    name: drift3

instead of this (basic/minimal implementation)

kind: Line
line:
- kind: Drift
  length: 0.25
  name: drift1
- MagneticMultipoleP:
    Bn1: 1.0
  kind: Quadrupole
  length: 1.0
  name: quad1
- kind: Drift
  length: 0.5
  name: drift2
- MagneticMultipoleP:
    Bn1: -1.0
  kind: Quadrupole
  length: 1.0
  name: quad2
- kind: Drift
  length: 0.5
  name: drift3

I think it should be possible to achieve the same by implementing element custom wrappers as well, as opposed to custom I/O. I could implement this alternative solution in a separate PR and then we could compare the two solutions and decide if we want to go forward with any of these.

It still seems to me like customization that could be postponed and/or avoided altogether.