Add NYGrid unit test

A unit test for NYGrid has been created to verify instantiation and simple attributes checks. The test function 'test_nygrid_obj' is implemented to ensure that the NYGrid can be correctly instantiated, and the start and end datetime are set correctly.

tests/test_nygrid.py

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+import os
+from datetime import datetime
+from nygrid.nygrid import NYGrid
+import pytest
+
+# Set up directories
+cwd = os.getcwd()
+if 'tests' in cwd:
+    parent_dir = os.path.dirname(cwd)
+    data_dir = os.path.join(parent_dir, 'data')
+else:
+    data_dir = os.path.join(cwd, 'data')
+
+grid_data_dir = os.path.join(data_dir, 'grid')
+if not os.path.exists(grid_data_dir):
+    raise FileNotFoundError('Grid data directory not found.')
+
+start_datetime = datetime(2018, 1, 1, 0, 0, 0)
+end_datetime = datetime(2018, 1, 2, 0, 0, 0)
+
+
+def test_nygrid_obj():
+    nygrid_sim = NYGrid(grid_data_dir,
+                        start_datetime=start_datetime.strftime('%m-%d-%Y %H'),
+                        end_datetime=end_datetime.strftime('%m-%d-%Y %H'),
+                        dcline_prop=None,
+                        esr_prop=None,
+                        vre_prop=None,
+                        verbose=True)
+
+    assert nygrid_sim is not None
+    assert isinstance(nygrid_sim, NYGrid)
+    assert nygrid_sim.start_datetime == start_datetime
+    assert nygrid_sim.end_datetime == end_datetime