fix #549 stderr formatting

[dbrakenhoff]

• add na_rep to _table_formatter_stderr, default is blank (for tables in plots) but use "nan" for fit_report.
• use stderr formatter in fit_report

Short Description

use existing stderr formatter in fit_report to avoid extremely large values being represented in normal notation.

Checklist before PR can be merged:

• closes issue [ENHANCEMENT] Improve formatting of "stderr" throughout Pastas #549
• is documented
• Format code with Black formatting
• type hints for functions and methods
• tests added / passed

[codacy-production]

Coverage summary from Codacy

Merging #580 (19d2986) into dev (1379345) - See PR on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: +0.00%)	100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (1379345)	4910	3565	72.61%
Head commit (19d2986)	4910 (+0)	3565 (+0)	72.61% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#580)	6	6	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[raoulcollenteur]

Looks good, merging! Thanks for fixing this long standing issue :)