linalg: export numeric constants

[perazz]

Motivation

As pointed out in #902, it is often tedious to read through templated numeric constants through the code.

Prior Art

#800

Additional Information

• For the user API, maybe all needed is a consistent way to export them? One way this is present in stdlib (codata module) is to use a mold parameter and function.
• For development (linear algebra routines will be easier to read), maybe the best approach would be to template them directly in fypp? this works well:

#! Return the unitary numeric constant for a given type and kind
#!
#! Args:
#!   type (string): an intrinsic type (complex, real, integer)
#!
#! Returns:
#!   Parameter value string
#!
#! E.g.,
#!   one('complex(dp)')  -> (1.0_dp, 0.0_dp)
#!   one('real(sp)') -> 1.0_sp
#!   one('integer') -> 1
#!
#:def one(type)
  #:assert 'integer' in type or 'real' in type or 'complex' in type
  #:if '(' in type and ')' in type
    #:set kind="_" + type.split('(')[1].split(')')[0]
  #:else
    #:set kind=""
  #:endif
  #:if 'integer' in type
    ${"1"+kind}$
  #:elif 'real' in type
    ${"1.0"+kind}$
  #:elif 'complex' in type
    ${"(1.0"+kind+",0.0"+kind+')'}$
  #:endif
#:enddef