New element for mileage change modelling
## Description
The mileage cange shall be modelled explicitly with a new funtional infrastructure element. This solution shall replace / complement the approach of RTM based `<linearPositioningSystem>` and its anchor points.
### Background
The request resulted from discussions in the railML ETCS use case working group.
### Links
* Forum discussion:
* Karl-Friedemann Jerosch, 16.09.2021: [https://www.railml.org/forum/index.php?t=msg&th=829&start=0&](https://www.railml.org/forum/index.php?t=msg&th=829&start=0&)
* Heidrun Jost, 01.10.2021: [https://www.railml.org/forum/index.php?t=msg&th=829&goto=2834&#msg_2834](https://www.railml.org/forum/index.php?t=msg&th=829&goto=2834&#msg_2834)
* Git issues:
* #478
* Wiki documentation:
## Proposed solution railML 3.2
New container element `<mileageChanges>` shall be added inside `<functionalInfrastructure>`. It includes a list of `<mileageChange>` elements.
The `<mileageChange>` has an attribute `@type` with values "gap", "overlap".
The `<mileageChange>` references child elements `<spotLocation>` via attributes `@from` and `@to`.
The `<mileageChange>` has two child elements `<spotLocation>`. One provides the link to the "incoming" mileage and the other one provides the link to the "outgoing" mileage.
issue