Use Pint and unit-free variable names in concrete-beam

Convert input parsing to Pint (matching the other calculations), carry
units in the YAML values, and drop unit suffixes from input keys, local
variables, results keys, the Typst sheet, and tests.

concrete-beam/input.yaml

@@ -1,15 +1,15 @@
 project: "Deer Creek Shoring"
 prepared_by: "Conemco Engineering"
 
-span_ft: 16
-tributary_ft: 6.25
-D_psf: 55
-L_psf: 20
-bw_in: 8
-h_in: 12
-d_in: 9.5
-fc_ksi: 3.0
-fy_ksi: 60
-As_in2: 0.62
-concrete_pcf: 150
+span: "16 ft"
+tributary: "6.25 ft"
+D: "55 psf"
+L: "20 psf"
+bw: "8 in"
+h: "12 in"
+d: "9.5 in"
+fc: "3.0 ksi"
+fy: "60 ksi"
+As: "0.62 in**2"
+concrete_density: "150 pcf"
 load_combination: "1.2D + 1.6L"