minor changes #331:
 - use mu as coefficient
 - define right hand side, default f = 1
update #331 and #282:
 - initialize element from expression
no need for shape, it can be deduced #331
update #300 use Lato font
remove sqwiggle
update #301
 - move element implementation in detail/element_impl.hpp
 - update thch
update #331
 - added evaluate_type
 - cleanup