Add XSPEC model string macro (closes #187)

[aditya-pandey-dev]

Implements bidirectional translation between XSPEC model strings and
XSPECModels.jl types, as requested in #187.

Changes

• lib/XSPECModels/src/xspec_string.jl — new file:

  • parse_xspec_model_string(str): runtime XSPEC string → Julia Expr
  • xspec"..." string macro: compile-time, zero overhead
  • xspec_model_string(model): reverse direction
  • XSPEC_MODEL_NAMES: 20 verified model mappings

• lib/XSPECModels/src/XSPECModels.jl — include + export new API

• lib/XSPECModels/test/test-xspec-string.jl — full test suite

• lib/XSPECModels/test/runtests.jl — include new tests

• docs/src/transitioning-from-xspec.md — complete guide for XSPEC users

• docs/make.jl — add page to navigation

• docs/src/models/xspec-models.md — mention new feature

• running_example_xspec_strings.jl — self-contained demo

Example

model = xspec"phabs*powerlaw"
model = xspec"tbabs*(diskbb+powerlaw+gaussian)"

xspec_model_string(XS_PhotoelectricAbsorption() * XS_PowerLaw())
# "phabs*powerlaw"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.49%. Comparing base (62aba68) to head (8b69229).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #242      +/-   ##
==========================================
+ Coverage   55.69%   57.49%   +1.79%     
==========================================
  Files          39       39              
  Lines        2632     2550      -82     
==========================================
  Hits         1466     1466              
+ Misses       1166     1084      -82     

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[aditya-pandey-dev]

@fjebaker I have raise a PR related to this issue can you please review the PR

[fjebaker]

Hello @aditya-pandey-dev thanks for your PR! I am away at a conference this week so it will take me some time to review this.