Restructure im-calc onto dask and annotate its output

[lispandfound]

im-calc read the whole broadband into memory as float64 and fanned the
IM kernels out over a process pool. That caps the station count at what
one node's RAM holds, and the pool is redundant now that the kernels in
im-calculation are dask-native.

Instead, open the broadband chunked over stations only -- component
and time must each stay whole, as the kernels take them as core
dimensions -- and let each kernel return a lazy Dataset. Nothing is
computed until to_netcdf, which streams chunk by chunk. --cores and
--psa-step are gone; dask sizes chunks from array.chunk-size.

Output becomes a DataTree with one group per intensity measure rather
than a flat Dataset, which is what lets each IM carry its own components
and its own units. Every leaf is annotated: IM_UNITS on the measures,
COORDINATE_METADATA on the station coordinates, so the file describes
itself instead of relying on the reader to remember that pSA is in g and
rrup in km.

The distance block moves out of the command body into Distances /
calculate_distances unchanged -- rrup, rjb, hyp and epi are
bit-identical to before. One deliberate change: rx and ry are now
divided by 1000 like every other measure. They were previously emitted
in metres while documented and grouped with distances in km.

calculate_source_parameters collapses a multi-fault realisation to the
single rake, dip, ztor and zbot a rupture is described by, moment-
weighted, and hangs them off the root attributes. Rakes are averaged as
unit vectors (Rakes.as_vectors), since the mean of -179 and 179
degrees is 0, not 180.

lat/lon are normalised to latitude/longitude on read, so the
command accepts SW4 low-frequency files as well as EMOD3D ones.

Needs the dask-native kernels from im-calculation's no_parallel
branch; pinned here until that is released.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com

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[github-actions]

Please change the following dependencies for consistency.

[github-actions]

Suggested change

	"im-calculation @ git+https://github.com/ucgmsim/IM_calculation@no_parallel",
	"im-calculation @ git+https://github.com/ucgmsim/IM_calculation.git@no_parallel",