Add non pure-multiplicative conversions

[szaghi]

For example support temperature-like conversions with offset, e.g.

• celsius = 273.15 + 1 * K
• fahrenheit = 255.372222 + 1 * K
• american_wire_gauge = 8.62491 * log(1/mm) + 18.2019 * mm
• dBm = 10 * log(mW)
• mW = 10**(dBm / 10)

supported conversions

• multiplicative-like
  • pure multiplicative: y = f * x
  • fixed_offset + multiplication: y = offset + f * x
• non multiplicative
  • generic f(x) user provided

how to implement generic f(x) user-provided alias-conversion formula

The current alias-conversion formula is implemented into the most base uom_symbol class that looks like

type :: uom_symbol
    integer :: exponent
    real :: factor
    real :: offset
    type(string) :: symbol
endtype uom_symbol

As a matter of fact, currently only multiplicative-like conversion are supported

direct: f(x) = offset + factor * x
inverse: f(x) = (x - offset) / factor

This should/could be generalized by a procedure pointer attribute customizable by user, e.g.

type :: uom_symbol
    integer :: exponent
    real :: factor
    real :: offset
    type(string) :: symbol
    procedure (generic_conversion_interface), pointer, nopass :: generic_conversion => null()
endtype uom_symbol

interface
    function generic_conversion_interface(x, inverse)
        real, intent (in) :: x
        logical, intent(in), optional :: inverse
        real :: generic_conversion_interface
    end function generic_conversion_interface
end interface

The uom_symbol gramar must be modified to take into account this new scenario, e.g.

grammar: @user A

that means that the symbol A has a conversion(s) formula supplied by user.

The problem is how the user can supply these formula, uom_symbol being the most basic (almost hidden to user) class...

[szaghi]

Done in new 0.8.3