docs: create documentation for emulator module

docs/source/emulator/index.rst

+Emulator
+========
+
+This module hosts functionality for emulation methods.
+
+Currently implemented classes are:
+
+* :class:`.ScalarGaSP`:
+
+* :class:`.PPGaSP`:
+
+.. toctree::
+   :maxdepth: 1
+   :hidden:
+   :caption: Emulator
+
+    RobustGaSP <robustgasp>
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docs/source/emulator/robustgasp.rst

+RobustGaSP
+----------
+
+ScalarGaSP Class
+^^^^^^^^^^^^^^^^
+
+The :class:`.ScalarGaSP` class is imported by::
+    
+    from psimpy.emulator.robustgasp import ScalarGaSP
+
+Methods
+*******
+.. autoclass:: psimpy.emulator.robustgasp.ScalarGaSP
+    :members: train, predict, sample, loo_validate
+
+
+PPGaSP Class
+^^^^^^^^^^^^
+
+The :class:`.PPGaSP` class is imported by::
+    
+    from psimpy.emulator.robustgasp import PPGaSP
+
+Methods
+*******
+.. autoclass:: psimpy.emulator.robustgasp.PPGaSP
+    :members: train, predict, sample
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