Revisit xmls - cdscan optimization and investigation of psql as alternative

[chaosphere2112]

As discussed earlier by @doutriaux1 @williams13 @painter1 there is a need to revisit cdscan as it's started to become difficult to maintain.

As noted in #124 additional to cdscan the psql package by Charlie O'Connor (https://github.com/UV-CDAT/uvcdat/tree/master/Packages/psql) may also be an approach to revisit when rethinking xmls.

@doutriaux1 @williams13 @painter1 - I think it's better to have a dedicated issue here, rather than tacking comments onto a bugfix issue for cdscan.

@doutriaux1 is 2.4 a likely milestone?

Migrated from: CDAT/cdat#1059