Only allocate real-space arrays when output is requested.

[quantumsteve]

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The size of real-space arrays may be larger than INT_MAX, causing an assertion error. Since these arrays are for an optional output, when it isn't needed we can just set their size to zero.

I also promoted the variable in dense_space_size to int64_t and check its size before returning.

This fixes #654

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[stefan-schnake]

We should consider removing the real_space features. It's not reasonable that you would want full phase space data. Maybe when the PDE rewrite is finished we could turn this into functionally that plots certain moments.