Add API for finding the first non-zero entry in a matrix row

[fingolfin]

In GAP we have an API PositionNonZeroInRow(mat, row) which returns the index of the first entry in the row row, or if the row only consists of zeros, then it returns ncols(mat)+1.

I think we should have a similar API in AA (it could do something different for zero rows, and e.g. return nothing).

It is a useful primitive on which various other efficient baseline methods for MatElem can be based on, e.g. for is_zero_row, is_zero, is_one, is_diagonal, is_upper_triangular, is_lower_triangular, ...

I.e. these methods would not have to fetch each entry (materializing matrix entries can cause allocations), and for e.g. compressed GF(2) matrices (where each entry is stored as a bit) they can scan entire machine words at once, instead of going bit-by-bit.

[fingolfin]

In Julia we could e.g. call this find_first_nonzero or so.

I forgot to mention: actually, one wants a three argument version. To explain, in GAP PositionNonZeroInRow(mat, row, after_col) ignores the entries in columns 1 to after_col, so it first looks in column after_col + 1, and returns a value > after_col.

This allows finding the second, third, ... etc. entry. The two-argument version then is just implemented for convenience, by using after_col = 0.

This is akin to Julia's findnext, though I personally am not a fan of this first/next split (but if people prefer that, I can deal with it).

I note that the convention in Julia is different, for findnext instead of after_col a "start index" is given, i.e. the first match will be after that, in general.

Both conventions have their merits, I am open to either