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?

changed milestone to %6.4.3

added workflowin development label

changed the description

added 4 commits

• 8e8569c2...655f8e0a - 2 commits from branch ogs:master
• 5930ba3f - [MPL] Reimplement VariableArray as own class
• 0196da72 - Update VariableArray access

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added workflowplease review label and removed workflowin development label

changed the description

changed the description

added 1 commit

• 6e8a4be2 - [PL/TRM] Remove double assignment

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added 11 commits

• 6e8a4be2...23cae104 - 7 commits from branch ogs:master
• 24770964 - [MPL] Reimplement VariableArray as own class
• 725339c0 - Update VariableArray access
• 897223e7 - [PL/TRM] Remove double assignment
• 3ba8d6cf - [MPL] Remove unused 'displacement' from variables

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added 6 commits

• 8b8acafe - [MPL] Reimplement VariableArray as own class
• 7a72b948 - Update VariableArray access
• 7f84722f - [PL/TRM] Remove double assignment
• fc2c63ab - [MPL] Remove unused 'displacement' from variables
• bbf26736 - [MPL] Remove unused typedefs. Shrink VariableType
• 1d19cd0a - [MPL] Remove VariableArray int access

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added 20 commits

• 1d19cd0a...ca8f05ce - 14 commits from branch ogs:master
• 7d2a2fd0 - [MPL] Reimplement VariableArray as own class
• e063c60b - Update VariableArray access
• cf1f5b97 - [PL/TRM] Remove double assignment
• 19c36bdd - [MPL] Remove unused 'displacement' from variables
• 2f8d25d0 - [MPL] Remove unused typedefs. Shrink VariableType
• 9f402f66 - [MPL] Remove VariableArray int access

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resolved all threads