data selection improvement (from CAS-2820)

[Kana Sugimoto]

Enable, e.g., scans = "1~5,8~10", in scantable selection. Currently user should set scans = [1,2,3,4,5,8,9,10] for equivalent selection and it's cumbersome.

[Kana Sugimoto]

Date: 2012/07/31
Rev.: 2611
Codes: scantable.py

Added a new method, scantable.parse_idx_selection(mode, expr), in scantable class to enable CASA type selection syntax of SCANNO, IFNO, POLNO, BEAMNO and row IDs in scantable.
This method parses a comma separated selection expression (expr) in a string to a list of IDs to set to selector. Index ranges in scantable are taken into account when parsing.
Available modes are, 'scan' (parses SCANNO selection), 'if' (prases IFNO selection), 'pol' (parses POLNO selection), 'beam' (parses BEAMNO selection), and 'row' (parses row id selction).

Usage:
stab = asap.scantable("SCANTABLE_NAME")
stab.scannos() # assume this returns successive numbers from 2 to 20.
scanlist = stab.parse_idx_selection("scan","<5,10~13,>18")
# returns [2,3,4, 10,11,12,13, 19,20]
stab.set_selection(scans=scanlist)

Also made "<=" and ">=" available in scantable._parse_selection().