Add filterRows/filterCols functions.

[drmoose]

This PR adds shortcut functions to the Matrix interface inspired by the filterKeys/filterValues functions supplied by Kotlin's Maps.

• filterRows
• filterRowsIndexed
• filterCols
• filterColsIndexed

All four take a predicate, and return a matrix that includes all of the (rows/cols) from the source matrix for which the predicate returns true.

>>> println(eye(6).filterRows { it[1] == it[3] }.repr())
mat[ 1.00,  0.00,  0.00,  0.00,  0.00,  0.00 end
     0.00,  0.00,  1.00,  0.00,  0.00,  0.00 end
     0.00,  0.00,  0.00,  0.00,  1.00,  0.00 end
     0.00,  0.00,  0.00,  0.00,  0.00,  1.00 ]

[kyonifer]

Looks good, thanks.