Savetxt unit

[fiolj]

This PR aims to add optional arguments to savetxt, that behave similar to numpy's savetxt.

This is associated with Issue 263 and this discussion thread.

It add the possibility of supplying the unit of an open file instead of a filename (which could be used for output_unit for instance)

This implementation is quite simple. The main changes are:

1. Add optional header and footer (that could be commented out with a character, currently defaults to '#')
2. Changes delimiter to an arbitrary length string. I am not completely sure that is really useful. Initially I've added this because np.savetxt (Numpy's) allows character or arbitrary strings but np.loadtxt requires it to be a length 1 char.
3. Add the possibility to override default format for saving data

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.64%. Comparing base (17a320a) to head (3f6aff3).

Files with missing lines	Patch %	Lines
example/io/example_savetxt.f90	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1085      +/-   ##
==========================================
- Coverage   68.67%   68.64%   -0.04%     
==========================================
  Files         393      393              
  Lines       12720    12724       +4     
  Branches     1376     1376              
==========================================
- Hits         8736     8734       -2     
- Misses       3984     3990       +6     

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[jalvesz]

Thanks for this @fiolj. Would you mind reverting the changes not related to the implementation? (styling) There are too many and it renders difficult to read through the PR. You can check the style_guide for info https://github.com/fortran-lang/stdlib/blob/master/STYLE_GUIDE.md

One thing, white spaces in-between parentheses and an intrinsic function are not recommended ( open (...), allocate (...), etc )

[fiolj]

Thanks @jalvesz, I've fixed the formatting