Add print methods for modeling functions

[Sean Kent]

At least need the following print methods

• print.misvm
• print.mildsvm
• print.smm

Consider other print methods for things like

• print.kfm_nystrom
• print.kfm_exact

[Sean Kent]

Want additional print methods for

• print.mior
• print.omisvm
• print.misvm_orova
• print.svor_exc
• print.mild_df

[Sean Kent]

Print methods for mild_df and mi_df were added in #49

[Sean Kent]

Done with #56