io: write out lp file with sliced variables and constraints

[FabianHofmann]

When writing out an LP file, large variables and constraints are now chunked to avoid memory issues. This is especially useful for large models with constraints with many terms. The chunk size can be set with the slice_size argument in the solve function.

@koen-vg I think we are merging this quite soon, could you have a try with your large pypsa-eur myopic network? I hope this will finally fix the issues.

[koen-vg]

I would love to try this out, at a conference now so I can't promise to be very quick about it, but I'll report back as soon as I've tested this.

[FabianHofmann]

@koen-vg thanks, sure, take your time. @lkstrp merging this as non-intrusive and well tested