fix: get_edge_ids() error for 2x2 matrices

[schochastics]

The function now throws a warning and indicates how the matrix is interpreted for backward compatibility

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[krlmlr]

I see that only three packages are affected: corpustools, mwcsr and SEMID. One of the results also hints at a misuse from igraph itself. I still think we should break cleanly and perhaps improve the error message.

[krlmlr]

Another alternative: disallow matrices altogether, ask users to convert matrices with c() or to pass data frames if they have data in columns.

[schochastics]

Fine by me, lets find a way in this PR

[krlmlr]

Let's await revdepchecks in #1793.

[maelle]

The second message (one row per edge) is clearer. Could the two be combined? My suggestion below is very bad though.

Suggested change

	details = c("For backward compatibility, edges are interpreted columnwise.", "convert to a data.frame with two columns and one row per edge.")
	details = c("For backward compatibility, it is assumed that each row is an edge, from the first column to the second column.", "convert to a data.frame with two columns and one row per edge.")