Additional PC2 optimisations for NG-ARCH

### Contact Details

@tinyendian

### Are there any linked Issues or Pull Requests?

ticket:1046
#53

### Brief description

This issue covers additional PC2 optimisations for the NG-ARCH project. The following subroutines will be optimised by adding OpenMP and compiler directives using PSyclone transmute scripts:
* `pc2_bm_initiate`
* `pc2_bl_forced_cu`
* `pc2_initiation_ctl`

### Further details of the issue.

Subroutine `pc2_bm_initiate` was originally targeted in TRAC ticket [#942](https://code.metoffice.gov.uk/trac/lfric_apps/ticket/942), but the PSyclone transmute script had to be excluded when the changes were merged as the removal of compiler directives through PSyclone transmutation led to KGO comparison failures.

This issue was subsequently addressed by TRAC ticket [#1046](https://code.metoffice.gov.uk/trac/lfric_apps/ticket/1046), which also includes the removal of an OpenMP Single region. The TRAC ticket will be completed with this GitHub issue, and extended with two additional PC2 optimisations to complete the NIWA/ESNZ contributions to NG-ARCH.

Start with the PSyclone transmute scripts for pc2_bm_initiate, pc2_bl_forced_cu, and pc2_initiation_ctl, then review the KGO comparison failures described for TRAC ticket 942. Apply the planned OpenMP and compiler-directive changes, and confirm that the relevant KGO comparisons pass without regressions.

Written by the indexing model from the issue text.