Defer matplotlib initialization for text rendering.

Rather than querying for matplotlib when
vtkMatplotlibMathTextUtilities is constructed, check only when
the library is needed. This saves some start up time for e.g.
ParaView when mathtext is not used.

Some additional changes are required due to a change in behavior:
vtkMatplotlibMathTextUtilities::New() used to return NULL when
matplotlib was not found to signal that mathtext was unavailable.
The vtkMathTextUtilities::IsAvailable() virtual method has been
added to allow querying of the availability, now that New() can
return a valid object when matplotlib is unavailable.