Added an is_string function since it string test is wodely used
Added an is_string function since it string test is wodely used

ghisvail/vispy,michaelaye/vispy,kkuunnddaannkk/vispy,Eric89GXL/vispy,julienr/vispy,drufat/vispy,srinathv/vispy,sbtlaarzc/vispy,sh4wn/vispy,sh4wn/vispy,RebeccaWPerry/vispy,inclement/vispy,bollu/vispy,kkuunnddaannkk/vispy,inclement/vispy,bollu/vispy,jdreaver/vispy,RebeccaWPerry/vispy,dchilds7/Deysha-Star-Formation,jdreaver/vispy,RebeccaWPerry/vispy,hronoses/vispy,michaelaye/vispy,ghisvail/vispy,Eric89GXL/vispy,sbtlaarzc/vispy,drufat/vispy,jay3sh/vispy,drufat/vispy,jay3sh/vispy,QuLogic/vispy,hronoses/vispy,inclement/vispy,Eric89GXL/vispy,ghisvail/vispy,michaelaye/vispy,sh4wn/vispy,dchilds7/Deysha-Star-Formation,julienr/vispy,QuLogic/vispy,srinathv/vispy,jdreaver/vispy,dchilds7/Deysha-Star-Formation,jay3sh/vispy,kkuunnddaannkk/vispy,bollu/vispy,srinathv/vispy,QuLogic/vispy,hronoses/vispy,julienr/vispy,sbtlaarzc/vispy