Add alias for cleaner use with FetchContent

[robertodr]

Provides an alias to the library so that use with FetchContent or imported target looks the same: target_link_libraries(foo PRIVATE XCFun::xcfun)

Status

• [x] Ready to go

[codecov-io]

Codecov Report

Merging #91 into master will decrease coverage by 2.89%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #91     +/-   ##
=========================================
- Coverage   44.97%   42.07%   -2.9%     
=========================================
  Files          78       77      -1     
  Lines        1839     1785     -54     
=========================================
- Hits          827      751     -76     
- Misses       1012     1034     +22

Impacted Files	Coverage Δ
src/fortran.c	0% <0%> (-65.96%)	:arrow_down:
src/densvars.hpp	28.33% <0%> (-7.23%)	:arrow_down:
src/xcfun.cpp	50% <0%> (-0.24%)	:arrow_down:
api/xcfun.F90

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[robertodr]

I caught a mismatch between the Autocmake setting for Fortran support (OFF by default) and in the bare CMake (ON by default) The last commit reconciles the two. I suggest a squash-merge.

[robertodr]

I'm cleaning up some last stuff. Apologies for the confusing signals.

[robertodr]

Ready to go, with a squash-merge.