ENH: Controlling how double items are displayed

Using ItemViews you can now control how the double value item is displayed based using the following "attributes":

* Notation - general display behavior.  Supported values include:
 * Fixed - displays the value in fixed notation.  This is equivalent to printf's %f flag
 * Scientific - displays the value in scientific notation.  This is equivalent to printf's %e flag
 * Mixed - tries to determine the best notation to use.  This is equivalent to printf's %g flag
* Precision - controls the precision (in the case of Fixed and Scientific Notations) or significant digits (in the case of Mixed Notation) that are to be displayed when the value is not being edited.
* EditPrecision - controls the precision (in the case of Fixed and Scientific Notations) or significant digits (in the case of Mixed Notation) that are to be displayed when the value is being edited.

Example SBT Code:

```xml
    <View Type="Instanced" Title="General">
      <InstancedAttributes>
        <Att Name="numerics-att" Type="numerics">
          <ItemViews>
            <View Item="dt_init" Type="Default" Precision="6" EditPrecision="10"/>
            <View Item="dt_max" Type="Default" Precision="6" EditPrecision="10" Notation="Fixed"/>
            <View Item="dt_min" Type="Default" Precision="6" EditPrecision="10" Notation="Scientific"/>
          </ItemViews>
        </Att>
        <Att Name="outputs-att" Type="outputs" />
        <Att Name="simulation-control-att" Type="simulation-control" />
<!--         <Att Name="Mesh" Type="mesh" /> -->
      </InstancedAttributes>
    </View>
```
13 jobs for setNumberOfSigDigits
Status Job ID Name Coverage
  External
passed cdash

passed ghostflow-branch-check-master

failed smtk@master-kerbin-windows-shared-release+examples+remus

00:01:07

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+private+remus

00:03:24

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+python3+remus+vtk

00:00:05

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+remus

00:03:30

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+remus+vtk

00:03:27

passed smtk@master-norkit-osx-shared-release+examples+matplotlib

00:01:28

failed smtk@master-praxis-windows-shared-release

03:42:39

passed smtk@master-talosiv-osx-shared-release+examples+matplotlib

00:02:31

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib

00:01:56

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib+private

00:02:30

passed cdash