[query] Add query_matrix_table an analogue to query_table

[chrisvittal]

CHANGELOG: Add query_matrix_table an analogue to query_table

Part of #14499.

Security Assessment

Delete all except the correct answer:

• This change has no security impact

Impact Description

Increases the query python API surface, but is only a query change.

[chrisvittal]

@patrick-schultz I'm getting an NPE in initialization, I think it's because this assertion in PartitionReader is running before zippedReader is initialized. Thoughts on resolving that?

hail/hail/hail/src/is/hail/expr/ir/IR.scala

Lines 1300 to 1301 in ecaa96b

	abstract class PartitionReader {
	assert(fullRowType.hasField(uidFieldName))

[patrick-schultz]

That's annoying. My first thought is to make the sub-readers lazy vals. Maybe try that for now, and we can think more if there's any more satisfying fix.

[chrisvittal]

Thanks for the suggestion. It worked. I guess lazy val can force initialization, but val is only truly valid after full initialization of the class.

[patrick-schultz]

Right, lazy val foo = init is basically private var _foo = null and def foo = { if (_foo == null) _foo = init; _foo }. So it will be initialized the first time it's accessed, rather than when the class initializer is run. Usually that means it's initialized later than the class, but in this case it happens earlier.