Molecular CR model implementation

[holm10]

Implementation of collisional-radiative model for hydrogen molecules, including molecular particle and energy rates.

• Molecular CRM model is controlled by switch bbb.ismolcrm
• Model only active is ishymol
• Model only reads CR rate files when ismolcrm > 0 -> not necessary for backwards-compatibility
• Switch defaults to 0, using old molecular model when ishymol
• bbb.ishymol is a float, scaling linearly between the models
• CR model fully activated when ismolcrm=1, old model completely deactivated
• Model adds a dimensions to dissociation-related arrays, as molecular dissociation produces less than 2 atoms (depending on the local quasi steady-state equilibrium)
• Corresponding arrays for molecular particle and energy transfer into ions and atoms have been added
• Internal routine engbal has been updated to reflect these changes

To do:

• Resolve albedo-like recycling of atoms into atoms -> albedo-like recycling of atoms to atoms & molecules given factor
• Simplify control of recycled atomic/molecular fraction
• Provide (minimal) working example of simulation using the molecular CRM