This will allow alternative methods of storing mixture information for e.g. complex transport modelling

Also remove the "<thermo" part of the names of thermodynamics packages

New base class for fluid and solid thermo: veryBasicThermo
Base class for fluid thermo: basicThermo (derived from veryBasicThermo)
Base class for solid thermo: solidThermo (derived from veryBasicThermo)

Note in next commit basicThermo -> fluidThermo, veryBasicThermo -> basicThermo

(more general: timeSelector should only be used for postprocessing utils)