Solve models in parallel with multi processing #562

Job	Run time
Lint	34s
Tests (3.10, git+https://github.com/pastas/[email protected])	1m 50s
Tests (3.10, git+https://github.com/pastas/[email protected])	1m 54s
Tests (3.10, git+https://github.com/pastas/[email protected])	1m 54s
Tests (3.10, git+https://github.com/pastas/[email protected])	1m 55s
Tests (3.10, git+https://github.com/pastas/pastas.git@dev)	1m 42s
Tests (3.11, git+https://github.com/pastas/[email protected])	1m 50s
Tests (3.11, git+https://github.com/pastas/[email protected])	1m 43s
Tests (3.11, git+https://github.com/pastas/[email protected])	1m 32s
Tests (3.11, git+https://github.com/pastas/[email protected])	1m 55s
Tests (3.11, git+https://github.com/pastas/pastas.git@dev)	3m 0s
Tests (3.12, git+https://github.com/pastas/[email protected])	1m 35s
Tests (3.12, git+https://github.com/pastas/pastas.git@dev)	1m 33s
	22m 57s

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