Add printing available plans

[olsaarik]

This implements issue #23 . The printing is available through sccl.print_plans() or in the CLI with sccl plans list. Both print out a GitHub-compatible markdown table like this:

Machine	Collective	# machines	From	To	Protocol	Priority	Plan name
ndv2	alltoall	>=2	1 MB	infinity	Simple	0	call synthesize_ndv2_relay_alltoall
ndv4	allreduce	1	256 KB	20 MB	LL128	0	run ndv4_ring_allreduce
ndv4	alltoall	8,16,32	1 MB	32 MB	LL128	0	run ndv4_alltoall
ndv4	alltoall	64	1 MB	32 MB	LL128	0	run ndv4_alltoall
ndv4	alltoall	8,16,32	32 MB	infinity	Simple	0	run ndv4_alltoall
ndv4	alltoall	64	32 MB	infinity	Simple	0	run ndv4_alltoall