Variable abundance option is much slower than power-law option

I realized this when running a bunch of simulations, but haven't actually done any careful benchmarking. This is of course not surprising given the need to read multiple tables and do the lookup as compared to just computing the power-law value.

We should do some benchmarking on the variable as well as coronal (or photospheric) cases and see if there are places where we could speed things up. The current runtimes make doing many hundreds of ebtel++ simulations cumbersome.