Battery / submeter interaction fix plus associated cleanups

[chipbarnaby]

Description

1. Fix bug re submeter accounting of battery energy flows. This involved a 2nd submeter walk after battery flows are determined.
   Expanded regression test submeter to better exercise battery cases.
2. Removed non-functional input BATTERY btEndUse. Now battery flows are always assigned to enduse BT. Documentation updated. Regression tests bt_test and pv_bt_tests updated to eliminate use of btEndUse. Addresses issue METER issues #475.
3. Refined mtr_Validate1() to avoid false positive messages.
4. Renamed record.gud to record.r_status to allow more precise grepping.

[nealkruis]

Note: This has API changes. We should make users aware that btEndUse is no longer valid input.