feat: implement GAMMA LN (#344) (#376)

Fixes #344

models/gamma_ln/gamma_ln.dat

@@ -0,0 +1,18 @@
+a
+0	12.8018
+1	12.8018
+b
+0	0.572365
+1	0.572365
+c
+0	-3.41362e-11
+1	-3.41362e-11
+FINAL TIME
+0	1
+INITIAL TIME
+0	0
+SAVEPER
+0	1
+1	1
+TIME STEP
+0	1

models/gamma_ln/gamma_ln.mdl

@@ -0,0 +1,60 @@
+{UTF-8}
+a = GAMMA LN(10) ~~|
+b = GAMMA LN(0.5) ~~|
+c = GAMMA LN(1) ~~|
+
+********************************************************
+	.Control
+********************************************************~
+		Simulation Control Parameters
+	|
+
+INITIAL TIME = 0 ~~|
+FINAL TIME = 1 ~~|
+TIME STEP = 1 ~~|
+SAVEPER = TIME STEP ~~|
+
+\\\---/// Sketch information - do not modify anything except names
+V300  Do not put anything below this section - it will be ignored
+*View 1
+$-1--1--1,0,|12||-1--1--1|-1--1--1|-1--1--1|-1--1--1|-1--1--1|96,96,100,0
+///---\\\
+:L<%^E!@
+1:gamma_ln.vdfx
+4:Time
+5:FINAL TIME
+9:gamma_ln
+19:100,0
+24:0
+25:1
+26:1
+15:0,0,0,0,0,0
+27:2,
+34:0,
+42:1
+72:0
+73:0
+35:Date
+36:YYYY-MM-DD
+37:2000
+38:1
+39:1
+40:2
+41:0
+95:0
+96:0
+97:0
+77:0
+78:0
+102:0
+93:0
+94:0
+92:0
+91:0
+90:0
+87:0
+75:
+43:
+103:8,8,8,3,8
+105:0,0,0,0,0,0,0,0,0,0
+104:Courier|12||0-0-0|0-0-0|-1--1--1|0-0-255|192-192-192|-1--1--1

packages/cli/src/c/vensim.h

@@ -13,12 +13,13 @@ extern "C" {
 //
 // Vensim functions
 // See the Vensim Reference Manual for descriptions of the functions.
-// http://www.vensim.com/documentation/index.html?22300.htm
+// https://www.vensim.com/documentation/22300.html
 //
 #define _ABS(x) fabs(x)
 #define _COS(x) cos(x)
 #define _EXP(x) exp(x)
 #define _GAME(x) (x)
+#define _GAMMA_LN(x) lgamma(x)
 #define _IF_THEN_ELSE(c, t, f) (bool_cond(c) ? (t) : (f))
 #define _INTEG(value, rate) ((value) + (rate) * _time_step)
 #define _INTEGER(x) trunc(x)