[FITTING] Name, description and saving of custom functions
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Description
==================
Add a possibility to name, describe and save custom functions used for fitting.
LabPlot Version
------------------
<!-- Add text from the labplot executable between the ``` ``` block below: Help -> About LabPlot -->
```
LabPlot 2.11.80
Release build b88280f Jan 31 2025, 07:21:32
System:
Ubuntu 24.10
Locale:
English,United States (Decimal point ',', Group separator ' '
Number Settings:
Decimal point ',', Group separator ' ', Exponential 'E', Zero digit '0', Percent '%', Positive/Negative sign '+'/'-' (Updated on restart)
Architecture:
x86_64-little_endian-lp64
Kernel:
linux 6.11.0-14-generic
Components:
C++ Compiler: GNU
C++ Compiler Flags:
Cantor 24.12.1
CFITSIO 4.3.0
Discount 2.2.7
Eigen3 3.4.0
FFTW3 3.3.7
GSL 2.7
HDF5 HDF5 library version: 1.14.3
KSyntaxHighlighting 6.10.0
KUserfeedback 6.10.0
liborigin 3.0.2
Matio 1.5.21
MCAP
NetCDF 4.8.0
ORCUS 0.19.2
Poppler 24.08.0
Purpose 6.10.0
QADS
Qt MQTT 6.8.1
Qt SerialPort 6.8.1
Qt Svg 6.8.1
QXlsx
ReadStat
Vector BLF
```
issue