Enhancing Equations

- add init file
- add docs
- fixing bug System of equation, replace torch.stack with torch.hstack
- add tests

pina/equation/__init__.py

@@ -0,0 +1,12 @@
+__all__ = [
+    'SystemEquation',
+    'Equation',
+    'FixedValue',
+    'FixedGradient',
+    'FixedFlux',
+    'Laplace',
+]
+
+from .equation import *
+from .equation_factory import *
+from .system_equation import *