llama: string_split fix

[Xarbirus]

The change fixes parsing of strings that contain spaces.
The non-template version works correctly, but in the template version operator>> stops reading at a space (which may be contained in the model path, for example).

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[ngxson]

I'm wondering, can we also explicit disable the template<class T> version with std::enable_if? Something like this:

template<typename T>
static typename std::enable_if<std::negation<std::is_same<T, std::string>::value>, bool>::type

This is just to make sure that if someone accidentally call string_split without specifying <std::string>, then it never use the template version

[slaren]

The template is already specialized for std::string, so I don't think that's necessary. Personally, I would very much prefer to avoid unreadable SFINAE nonsense, a static_assert would do the same if absolutely necessary.

[Xarbirus]

Now we will not be able to call string_split without specifying std::string, because we will get an error like

error: no matching function for call to 'string_split'
note: candidate template ignored: couldn't infer template argument 'T'

But I added a static_assert in case the specialized version is accidentally removed.