Include statment in NESTML

[pnbabu]

Fixes #884.

Todo

• Add support for multiple blocks of the same type (multiple parameters, internals, equations, functions, onReceive and onCondition (this is already the case)--but not multiple update, input, output blocks) -- see Allow multiple state, parameter, internals, equations blocks in a model #1173.
• Allow docstring in included files (see Use single-line comment characters for docstrings #1169).

[github-actions]

Bencher Report

Branch	1121/merge
Testbed	ubuntu-latest

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Benchmark	Latency	nanoseconds (ns)
tests/nest_continuous_benchmarking/test_nest_continuous_benchmarking.py::TestNESTContinuousBenchmarking::test_stdp_nn_synapse	📈 view plot ⚠️ NO THRESHOLD	4,045,641,898.80

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