Add "arange" matrix creation

[konovod]

Add "arange" matrix creation

[jtanderson]

Update: after reading/experimenting with numpy.arange, this version follows it more closely, and is a bit friendlier in general:

    def self.arange(start_val : T, end_val : T, delta = 1.0)
      return GeneralMatrix(T).new(1,0) unless (end_val - start_val).sign == delta.sign && delta.abs > 0
      GeneralMatrix(T).new(1, ((end_val - start_val).abs.ceil / delta.abs).ceil.to_i) do |i,j|
        start_val + j*delta
      end
    end

One departure from numpy.arange is that if delta: 0 then an empty row will be returned. The numpy version just allows the inner division throw a ZeroDivisionError exception.

[jtanderson]

I have this code ready with tests and can submit a pull-request is anybody is around to approve!