Add Phase 2 content: High-Order Methods and Elastic Wave Equations

New chapter:
- Chapter 8: High-Order Methods
  - Dispersion Analysis (numerical dispersion, phase/group velocity)
  - Fornberg Algorithm (computing FD weights for arbitrary order)
  - Dispersion-Relation-Preserving (DRP) Schemes (Tam-Webb optimization)
  - Custom weights in Devito: u.dx2(weights=weights)

New section in Chapter 7:
- 7.3 Elastic Wave Equations
  - Velocity-stress formulation
  - VectorTimeFunction and TensorTimeFunction
  - Vector operators: div, grad, diag
  - Staggered grid discretization
  - Varying Lame parameters for heterogeneous media

Key Devito patterns introduced:
- Custom FD weights for optimized dispersion
- VectorTimeFunction for velocity fields
- TensorTimeFunction for stress tensors (symmetric=True)
- Vector calculus operators (div, grad, diag)
- Staggered grid handling for first-order systems

New solvers: dispersion.py, drp_devito.py, elastic_devito.py
New tests: 73 tests (39 highorder, 34 elastic)
Added references for Fornberg, Tam-Webb DRP schemes

Author: ggorman

.typos.toml

@@ -8,3 +8,5 @@ strang = "strang"
 Strang = "Strang"
 # Variable name for "p at iteration n" in Jacobi iteration
 pn = "pn"
+# NumPy function for creating array ranges
+arange = "arange"

_quarto.yml

@@ -21,6 +21,7 @@ book:
         - chapters/nonlin/index.qmd
         - chapters/elliptic/index.qmd
         - chapters/systems/index.qmd
+        - chapters/highorder/index.qmd
     - part: "Appendices"
       chapters:
         - chapters/appendices/formulas/index.qmd
@@ -173,6 +174,7 @@ src_trunc: "https://github.com/devitocodes/devito_book/tree/devito/src/trunc"
 src_advec: "https://github.com/devitocodes/devito_book/tree/devito/src/advec"
 src_elliptic: "https://github.com/devitocodes/devito_book/tree/devito/src/elliptic"
 src_systems: "https://github.com/devitocodes/devito_book/tree/devito/src/systems"
+src_highorder: "https://github.com/devitocodes/devito_book/tree/devito/src/highorder"
 src_formulas: "https://github.com/devitocodes/devito_book/tree/devito/src/formulas"
 src_softeng2: "https://github.com/devitocodes/devito_book/tree/devito/src/softeng2"