BGK: generalize initial conditions parser

[JamesMcClung]

Rewrote the parser for BGK initial conditions in a completely generalized way, without any hardcoded values. The new parser in table.hxx parses an arbitrary file with space-separated values, reading column headers from the first line and treating the remaining lines as data until it hits an empty line or EOF.

This implementation stores the data in a row-major order using vectors (so I suppose it isn't row-major in the same sense that a 2d array would be). There's one big vector containing the rows, which are also vectors. A column-major order might be significantly more performant, but row-major was more convenient for the parsing algorithm, and currently the table is only used during initialization. I almost rewrote it to be column-major, but I figured it should be unit tested first.

[germasch]

LGTM