update issue #239: refactor polynomialset.hpp
 - moved `update()` function to polynomialset_impl.hpp
 - do not inline `update()`
update #279: split functionpace instantiations even more
update #279: cleanup 
 - moved `exporter options()` to options.cpp 
 - removed `feel/feelfilters/exporter.cpp`
`feel options()` is now set automatically using `_desc_lib` (#273)