Create fits files

[pecom]

Adds functionality to save blended/deblended batches as FITS files.

[codecov[bot]]

Codecov Report

Attention: 93 lines in your changes are missing coverage. Please review.

Comparison is base (cef8222) 61.15% compared to head (ed30853) 57.96%. Report is 1 commits behind head on dev.

Files	Patch %	Lines
btk/blend_batch.py	9.70%	93 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #465 +/- ## ========================================== - Coverage 61.15% 57.96% -3.20% ========================================== Files 19 19 Lines 1555 1658 +103 ========================================== + Hits 951 961 +10 - Misses 604 697 +93 ``` | [Flag](https://app.codecov.io/gh/LSSTDESC/BlendingToolKit/pull/465/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/LSSTDESC/BlendingToolKit/pull/465/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC) | `57.96% <9.70%> (-3.20%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ismael-mendoza]

another question @pecom is the saving and loading with fits files useful to you for the DeblendBatch ? If not I would suggest not including it

[ismael-mendoza]

@pecom I'm closing this for now, is it finished? feel free to let me know or re-open if you are still interested