Parse non-scientific reals using exponents rather than factors

[timothy-nunn]

Description

Correct a difference in parsing decimal vs scientific notations in IN.DATs. By parsing decimals (e.g. 0.08) using exponents we mitigate a slight numerical inaccuracy that occured when sequentially multiplying by 0.1 (which Fortran cannot exactly represent).

Checklist

I confirm that I have completed the following checks:

• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[timothy-nunn]

Tests for 0.008 etc created using the following program

program test
    use, intrinsic :: iso_fortran_env, only: dp=>real64

    implicit none

    real(dp) :: a

    a = 8.0D-3

    print *, "a ", a
end program test

>  a    8.0000000000000002E-003

[timothy-nunn]

tests/unit/test_input.py has been added to show this fix actually works. On main, 1 failed and 29 passed. On this branch, all 30 pass.