Create errors.h for error diagnostics

[alecjohnson]

I want to check in two groups of error diagnostics methods, to be accessed via

  #include "errors.h"

Firstly, a generic error message utility:

  eprintf(char*, varargs);

This prints file, line number, function, and the error message, and then exits. Similarly, a generic warning utility:

  Wprintf(char*, varargs);

Secondly, a generic utility to use when no case is selected:

  invalid_value_error(int);
  invalid_value_error(char *);
  invalid_value_error(double);

and

  unsupported_value_error(int);
  unsupported_value_error(char *);
  unsupported_value_error(double);

The most typical use is in the default clause of a switch statement. For example, in a routine to compute based on the number of dimensions :

    switch(ndims)
    {
        default: unsupported_value_error(ndims);
        case 2: b2=1-mbc; s2=S2+2*mbc;
        case 1: b1=1-mbc; s1=S1+2*mbc;
        case 0: ;
    }