Add troubleshooting doc

[richfitz]

E.g., where lsoda requires more steps, as it's not obvious:

• where to look for help on this
• how to pass the additional args in

[richfitz]

this sort of thing

DLSODA-  At current T (=R1), MXSTEP (=I1) steps   
      taken on this call before reaching TOUT     
In above message, I1 = 5000
 
In above message, R1 = 23.204
 
Warning messages:
1: In lsoda(y, times, func, parms, ...) :
  an excessive amount of work (> maxsteps ) was done, but integration was not successful - increase maxsteps
2: In lsoda(y, times, func, parms, ...) :
  Returning early. Results are accurate, as far as they go

[richfitz]

now more fully discussed here: https://github.com/mrc-ide/odin/issues/243