relax SVN archive

Bug fix for the base class value setting method set().

If no default value exists, this is now caught and a RelaxError is thrown.

Updated the value.set() user function to use the correct J(w) mapping value setting docstring.

Renamed the J(w) mapping specific set() method to set_doc().

The base class method set() is used instead.

Updated the J(w) mapping code to handle a spin system specific 'nucleus' variable.

The specific value setting function, of the base class, now handles string values.

Fixed all the unit tests of setting the heteronucleus type.

The variable will be called 'nucleus' and not 'heteronuc'.

Removed all the setting of the homonucleus unit tests.

Updated the model-free code to handle a spin system specific 'nucleus' variable.

Wrote 12 unit tests for using the value setting functions to set the hetero and homonuclei.

This is in preparation for replacing the nuclei() user function with the more powerful value.set().
The values also need to now be spin specific.

Shifted all the gyromagnetic ratio values into the physical constants file.