Standardize names in `Stage.evolution` that stores beam parameter evolution through a stage

[ben-c-2013]

Currently, there are different namea used for storing beam parameter evolution data. E.g.

StageReducedModels.evolution.beam has

['beam_size_x',
 'beam_size_y',
 'beta_x',
 'beta_y',
 'bunch_length',
 'charge',
 'divergence_x',
 'divergence_y',
 'emit_nx',
 'emit_ny',
 'energy',
 'location',
 'num_particles',
 'plasma_density',
 'rel_energy_spread',
 'x',
 'x_angle',
 'y',
 'y_angle',
 'z']

while StageWakeT.evolution.beam has

['beam_size_x',
 'beam_size_y',
 'beta_x',
 'beta_y',
 'bunch_length',
 'charge',
 'emit_nx',
 'emit_ny',
 'energy',
 'energy_spread',
 'location',
 'plasma_density',
 'rel_energy_spread',
 'x',
 'xp',
 'y',
 'yp',
 'z']