Collection of small updates

• fix zone plate height and width definition. Closes #7 (closed)
• incorporate instruction and documentation in the notebook. Closes #6 (closed)
• add beam center calculation. Closes #8 (closed)
• provides install requirements. Closes #5 (closed)

src/TZPGcalc/TZPGcalc.py

@@ -715,7 +715,7 @@ class TZPGcalc():
             style=style,
             layout=layout
         )
-        self.widgets['TZPGwH'] = widgets.BoundedFloatText(
+        self.widgets['TZPGwV'] = widgets.BoundedFloatText(
             value=1.0,
             min=.1,
             max=3.0,
@@ -724,7 +724,7 @@ class TZPGcalc():
             style=style,
             layout=layout
         )
-        self.widgets['TZPGwV'] = widgets.BoundedFloatText(
+        self.widgets['TZPGwH'] = widgets.BoundedFloatText(
             value=1.0,
             min=.1,
             max=3.0,