Eliminate CompactLieAlgebra::SimpleAlgebra "to string" methods from API

[Nicholas Setzer]

The methods

to_string()
to_physics_string()
toMathML()

in CompactLieAlgebra::SimpleAlgebra are now superseded by the method get_symbol() and therefore should be removed from the API. It may thus be necessary to add a parameter to the get_symbol() to permit either A_N, B_N, etc or SU(N), SO(2N), etc to be returned.