Added doctests for read_asc_header.
Added doctests for read_asc_header.

landlab/landlab,csherwood-usgs/landlab,Carralex/landlab,ManuSchmi88/landlab,cmshobe/landlab,landlab/landlab,csherwood-usgs/landlab,Carralex/landlab,SiccarPoint/landlab,ManuSchmi88/landlab,amandersillinois/landlab,RondaStrauch/landlab,SiccarPoint/landlab,amandersillinois/landlab,cmshobe/landlab,Carralex/landlab,RondaStrauch/landlab,landlab/landlab,cmshobe/landlab,ManuSchmi88/landlab,laijingtao/landlab,RondaStrauch/landlab,laijingtao/landlab