fix: Set Row.insert_cols to accept AbstractMat

Row.insert_cols(..) was previously set to only accept Row vectors. This
is now corrected to accept any kind of matrix as long as its shape maps
to an column vector; which is checked inside the method.

Fixes #84

Signed-off-by: Sebastian Niemann <niemann@sra.uni-hannover.de>
fix: Fixed load error and file name conversion

Matrix files with an empty line at the beginning are now handled as entirely empty matrices.

Fixed #76 ()
Fixed #99 ()

Signed-off-by: Sebastian Niemann <niemann@sra.uni-hannover.de>