make_expmap.py: Threshold-cut the exposure map (cut=1.5%)
make_expmap.py: Threshold-cut the exposure map (cut=1.5%)

* Copy several keywords from instrument map (as 'fluximage' does)
* Set the merged expmap has block name "EXPMAP" (as 'fluximage' does)

liweitianux/chandra-acis-analysis,liweitianux/chandra-acis-analysis,liweitianux/chandra-acis-analysis