Ignore unneeded spins.conf options

Some options specified in spins.conf are unimportant to the conversion to netcdf. The conversion only requires the following parameters to be specified: Nx, Ny, Nz, Lx, Ly, Lz, min_x, min_y, min_z plot_interval type_x, type_y, type_z mapped_grid

All others can be ignored. There is a way for boost to ignore an input and do nothing about it. See http://www.boost.org/doc/libs/1_56_0/doc/html/program_options/howto.html#idp344642720 for more information.

This is an important feature because we have many cases that have different problem parameters which are unimportant to the conversion.

Please make the following have the given default values as well: Nx = 1, Ny = 1, Nz = 1, Lx = 1, Ly = 1, Lz = 1, (already the defaults) min_x = 0, min_y = 0, min_z = 0, (already the defaults) mapped_grid = false

If plot_interval or type_* are not in the spins.conf return an error asking for these to be specified.