Solve models in parallel with multi processing #560

Job	Run time
Lint	31s
Tests (3.10, git+https://github.com/pastas/[email protected])	1m 42s
Tests (3.10, git+https://github.com/pastas/[email protected])	1m 43s
Tests (3.10, git+https://github.com/pastas/[email protected])	1m 38s
Tests (3.10, git+https://github.com/pastas/[email protected])	1m 51s
Tests (3.10, git+https://github.com/pastas/pastas.git@dev)	1m 34s
Tests (3.11, git+https://github.com/pastas/[email protected])	1m 46s
Tests (3.11, git+https://github.com/pastas/[email protected])	1m 40s
Tests (3.11, git+https://github.com/pastas/[email protected])	1m 46s
Tests (3.11, git+https://github.com/pastas/[email protected])	2m 1s
Tests (3.11, git+https://github.com/pastas/pastas.git@dev)	2m 39s
Tests (3.12, git+https://github.com/pastas/[email protected])	1m 37s
Tests (3.12, git+https://github.com/pastas/pastas.git@dev)	1m 28s
	21m 56s

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