ENH: Adding Support for Units in SMTK

Expanding Units Support in Attribute Resource

SMTK's Attribute DoubleItems and DoubleItemDefinitions can now support units specified as Defaults as well as values. When a default's or value's units differ from those defined by the Item's Definition, they are converted into the Definition's units. If no conversion is possible then the method assigning the default or value will fail.

The Item's value(...) methods will always return the value in the units specified in its Definition. The Item's valueAsString(...) methods will always return a string based on the unconverted value

Developer changes

SMTK Resources can now hold a units::System.  In the case of an Attribute Resource, it will have a
default units::System associated with it at construction time; however, this can be replaced as long
as there are no Definitions defined within the Resource.

New Resource Methods:
* setUnitsSystem(const shared_ptr<units::System> & unitsSystem)
* const shared_ptr<units::System> & unitsSystem() const;

All ItemDefintions now hold onto a units::System.  The methods are protected and are identical to the ones added to Resource.

DoubleItemDefinition has the following new methods:

* bool setDefaultValue(const double& val, const std::string& units);
* bool setDefaultValue(const std::vector<double>& vals, const std::string& units);
* bool setDefaultValueAsString(const std::string& val);
* bool setDefaultValueAsString(const std::vector<std::string>& vals);
* const std::string defaultValueAsString(std::size_t element = 0) const;
* const std::vector<std::string> defaultValuesAsStrings() const;

DoubleItem has the following new methods:

* bool setValue(std::size_t element, const double& val, const std::string& units);

In addition, DoubleItem::setValueFromString method can now handle strings that include a double
followed by an option units.  For example "20 m/s".