Possible memory savings -- low use arrays

[David Dickinson]

Certain large arrays may always be allocated but not used often etc. This issue is meant to just record/document some of these which we may wish to remove.

1. vpar, supplied by dist_fn_arrays and only used in dist_fn. Only appears in init_invert_rhs and get_source_term (un-optimised source) or init_source_term (optimised source). We could easily replace vpar with direct calculation at the points of use.
2. vpac, similar to vpar (only used in dist_fn) but involves less calculation to form. Used a bit more than vpar. Grid centred vpa.
3. gexp_* arrays for certain split nonlinear approaches as these arrays are not used in some of these approaches.

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[David Dickinson]

With some proposed changes in PR #657 we may not be saving much calculation by the precomputation of coefficients in the A|| source term so we may prefer to save memory and revert to calculating these as needed.