Make scipy an optional dependency

[thomaspinder]

Problem

scipy is currently a required dependency, but it's only used by SignRestriction in identification.py (specifically scipy.stats.special_ortho_group for generating random rotation matrices).

Proposed Solution

1. Move scipy from dependencies to [project.optional-dependencies]:

   [project.optional-dependencies]
   sign = ["scipy>=1.10"]

2. Add a lazy import with a clear error message in SignRestriction:

   try:
       from scipy.stats import special_ortho_group
   except ImportError:
       raise ImportError(
           "scipy is required for sign restrictions. "
           "Install with: pip install litterman[sign]"
       )

3. Update documentation to mention the optional dependency.

Impact

Reduces the install footprint for users who only need Cholesky identification.

Labels

enhancement, dependencies