MPL: Reimplement VariableArray as own class

• Reducing the size of the VariableArray structure; new size is 408 bytes, old size is 1344 bytes.
• allowing less types/stricter types,
• simplified access to variables.

It also seems to be faster for at least one test: 4.138/4.345 \approx 95.2\%

• master:

Benchmark 1: bin/ogs ../s/Tests/Data/ThermoRichardsMechanics/PointHeatSource/point_heat_source_2D.prj
  Time (mean ± σ):      4.345 s ±  0.034 s    [User: 4.632 s, System: 0.154 s]
  Range (min … max):    4.264 s …  4.422 s    100 runs

• This MR:

Benchmark 1: bin/ogs ../s/Tests/Data/ThermoRichardsMechanics/PointHeatSource/point_heat_source_2D.prj
  Time (mean ± σ):      4.138 s ±  0.020 s    [User: 4.375 s, System: 0.150 s]
  Range (min … max):    4.094 s …  4.180 s    100 runs

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?