Provide precision-preserving edit descriptors

[milancurcic]

Motivation

In #505 @epagone proposed:

Considering that these formats are more useful than only for savetxt and loadtxt, would it make sense to make them available on their own in stdlib_io and simply use them? I am thinking, for example, of any sort of I/O where round-trip accuracy is required (e.g. writing to a CSV, JSON, TOML, YAML, etc...) to override a default behaviour that might be less strict.

Prior Art

Currently the formats used for savetxt and loadtxt are private to the stdlib_io and not usable from the outside:

stdlib/src/stdlib_io.fypp

Lines 22 to 32 in 57cfaf0

	! Format strings with edit descriptors for each type and kind
	character(*), parameter :: &
	FMT_INT = '(*(i0,1x))', &
	FMT_REAL_SP = '(*(es15.8e2,1x))', &
	FMT_REAL_DP = '(*(es24.16e3,1x))', &
	FMT_REAL_XDP = '(*(es26.18e3,1x))', &
	FMT_REAL_QP = '(*(es44.35e4,1x))', &
	FMT_COMPLEX_SP = '(*(es15.8e2,1x,es15.8e2))', &
	FMT_COMPLEX_DP = '(*(es24.16e3,1x,es24.16e3))', &
	FMT_COMPLEX_XDP = '(*(es26.18e3,1x,es26.18e3))', &
	FMT_COMPLEX_QP = '(*(es44.35e4,1x,es44.35e4))'

They work with scalars and arrays alike:

print FMT_REAL_DP, 1.23d0
print FMT_REAL_DP, [1.23d0, 4.56d0]

outputs

 1.2300000000000000E+000
 1.2300000000000000E+000  4.5599999999999996E+000

So it seems to me that the solution could be to make the existing constants public. @epagone @certik do you foresee any scenario where it wouldn't work?

Additional Information

@epagone would you like to submit a PR for this?

[certik]

I think this might work and should cover the use cases I had in mind. I initially thought to have just es15.8e2, but the full format string is even easier for the end user.

[epagone]

@milancurcic I am quite tight on time for the next few weeks but if it not urgent, I am happy to give it a try. On the other hand, if you or someone else wants to go ahead, I am happy too (I am eager to complete first a fpm PR already assigned to me).

PS: I know it's a trivial PR but my GitHub development skills are very rusty and I can use it only very occasionally so I need to re-learn all the command line steps every time, so the time investment in my case is non-trivial.