Added two methods for creating canonical MeasurementSets
- pyrap.tables.default_ms(name, table_description)
- pyrap.tables.msutils.required_ms_desc(table)

default_ms_subtable("ANTENNA") will create an ANTENNA subtable, for
example. Additional table descriptions can be added by calling
default_ms_subtable("ANTENNA", {...}) for example.

Empty HCcoordnames and HCidnames list aren't accepted by casacore, strip
them out of hypercolumn definitions so that the table description can be
reused when creating a table.

makedminfo infers the basic data manager outline from a tabledesc and
allows the user to supply SPEC dictionary entries for a Data Manager
group. In practice, this allows the user to set the following on Tiled
Storage Managers

- DEFAULTTILESIZE
- MAXIMUMCACHESIZE

where DEFAULTTILESIZE is likely most useful.

Remove the keyword from the column description.
This empty Vector<String> gets converted to a python dictionary as
'FLAG_CATEGORY' : {
     ...
     keywords': {'CATEGORY' : []},
     ...
}

Due to the missing type information this gets converted
into something like Vector<int> when passed back to the C++
layer, which results in Table Conformance errors.
This is a reasonable solution since the C++ layer always adds
this keyword if it is missing from the MS (see addCat())