autocorrelation pair count normalization

As pointed out by @Christopher-Bradshaw in #962 (and also https://github.com/manodeep/Corrfunc/pull/201), the `angular_tpcf` function appears to have an incorrect factor of Nr x Nr instead of Nr x (Nr-1). This has a negligible impact on all practical cases of interest, but the error is needless and should still be fixed. The main question concerns whether other `mock_observables` functions are impacted as well, or whether this is isolated to `angular_tpcf`.