Replace Zero_vector by vector{}

As far as the set of .cpp files which are using vecmat.h are
concerned, `Zero_vector` is out of reach for the compiler optimizer,
because it is extern / lives in a separate translation unit. An
expression like `x == Zero_vector` or `v = Zero_vector` thus has to
perform memory loads (for Zero_vector's x,y,z parts) before
comparison or copying, respectively. By using an immediate zero
vector `vector{}` instead, the unnecessary extra loads should go
away.

I present exhibit A:

```
void copyxx(vector *x) { *x = Zero_vector; }

4905e0: 48 8b 05 41 c0 56 00  movq 0x56c041(%rip),%rax  # 9fc628 <Zero_vector>
4905e7: 48 89 07              movq %rax,(%rdi)
4905ea: 8b 05 40 c0 56 00     movl 0x56c040(%rip),%eax  # 9fc630 <Zero_vector+0x8>
4905f0: 89 47 08              movl %eax,0x8(%rdi)
4905f3: c3                    ret
```

vs.

```
void copyxx(vector *x) { *x = vector{}; }

4905c0: 48 c7 07 00 00 00 00  movq $0x0,(%rdi)
4905c7: c7 47 08 00 00 00 00  movl $0x0,0x8(%rdi)
4905ce: c3                    ret
```

scripts/osiris_vector.h

@@ -22,8 +22,6 @@
 #include "fix.h"
 #include "vecmat_external.h"
 
-const vector Zero_vector = vector{};
-
 // Disable the "possible loss of data" warning
 #pragma warning(disable : 4244)