Small fix to Euler solver

Squash output message every time the restart file is written
by writing to default file name, rather than re-setting the name
every time.
User manual: small section on writing data files

How to create a new data file output, write the data, and read into
Python or IDL.
Adding timestep monitor to solvers

To monitor or save data after every timestep, a monitor can be added
to the solver. For this to work with PVODE & CVODE, they need to be run
in one_step mode, currently off by default. To enable, use the setting:

[solver]
single_step = true

(or command-line "solver:single_step=true")
Timestep monitor in new API

To enable, must set

[solver]
monitor_timestep = true

For CVODE and PVODE solvers, this switches to single step mode.

In the new API based on PhysicsModel, the monitors are protected
virtual functions which can be implemented by user code.

See examples:

monitor        : Old (C-like) API
monitor-newapi : New (OO) API
User manual section on monitors

Added brief description of timestep monitoring
Adding timestep monitor capabilities