Feat: channels var size helper

[lterrac]

Description

• feat: add helperfunction to compute payload buffer size in var size spsc channel

Type of change

What type of changes does your code introduce to HiCR? Put an x in the box that apply.

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

[github-actions]

Coverage amd64

Package	Line Rate	Branch Rate	Health
include.hicr.backends.boost	75%	43%	➖
include.hicr.backends.hwloc	77%	37%	➖
include.hicr.backends.mpi	92%	52%	✔
include.hicr.backends.nosv	71%	45%	➖
include.hicr.backends.pthreads	85%	50%	✔
include.hicr.core	72%	45%	➖
include.hicr.frontends.RPCEngine	100%	49%	✔
include.hicr.frontends.channel	95%	62%	✔
include.hicr.frontends.channel.fixedSize	100%	50%	✔
include.hicr.frontends.channel.fixedSize.mpsc.locking	92%	65%	✔
include.hicr.frontends.channel.fixedSize.mpsc.nonlocking	96%	57%	✔
include.hicr.frontends.channel.fixedSize.spsc	95%	68%	✔
include.hicr.frontends.channel.variableSize	95%	50%	✔
include.hicr.frontends.channel.variableSize.mpsc.locking	84%	50%	✔
include.hicr.frontends.channel.variableSize.mpsc.nonlocking	95%	48%	✔
include.hicr.frontends.channel.variableSize.spsc	93%	60%	✔
include.hicr.frontends.objectStore	63%	41%	➖
include.hicr.frontends.tasking	80%	56%	✔
Summary	83% (1968 / 2371)	48% (1138 / 2375)	✔

Minimum allowed line rate is 60%

[github-actions]

Coverage arm64

Package	Line Rate	Branch Rate	Health
include.hicr.backends.boost	75%	43%	➖
include.hicr.backends.hwloc	77%	37%	➖
include.hicr.backends.mpi	92%	52%	✔
include.hicr.backends.nosv	71%	45%	➖
include.hicr.backends.pthreads	85%	50%	✔
include.hicr.core	72%	45%	➖
include.hicr.frontends.RPCEngine	100%	48%	✔
include.hicr.frontends.channel	95%	62%	✔
include.hicr.frontends.channel.fixedSize	100%	50%	✔
include.hicr.frontends.channel.fixedSize.mpsc.locking	92%	65%	✔
include.hicr.frontends.channel.fixedSize.mpsc.nonlocking	94%	55%	✔
include.hicr.frontends.channel.fixedSize.spsc	95%	68%	✔
include.hicr.frontends.channel.variableSize	95%	50%	✔
include.hicr.frontends.channel.variableSize.mpsc.locking	84%	52%	✔
include.hicr.frontends.channel.variableSize.mpsc.nonlocking	95%	48%	✔
include.hicr.frontends.channel.variableSize.spsc	93%	60%	✔
include.hicr.frontends.objectStore	63%	41%	➖
include.hicr.frontends.tasking	80%	56%	✔
Summary	83% (1968 / 2372)	48% (1139 / 2379)	✔

Minimum allowed line rate is 60%