Added metadata to model, and included it in printout

[dlakaplan]

from pint.models import get_model
m=get_model("tests/datafile/NGC6440E.par")
print(m.as_parfile())

now returns:

# Created: 2024-12-12T13:41:07.865053
# PINT_version: 0.7+6065.g92675b80
# User: David Kaplan (kaplan)
# Host: plock
# OS: macOS-10.16-x86_64-i386-64bit
# Python: 3.9.7 (default, Sep 16 2021, 08:50:36) 
# [Clang 10.0.0 ]
# Format: pint
# read_time: 2024-12-12T13:41:04.663636
# allow_tcb: False
# convert_tcb: False
# allow_T2: False
# original_name: tests/datafile/NGC6440E.par
PSR                            1748-2021E
...

So there is now a metadata dictionary, which is initially populated with arguments that were used when reading in the par file along with the name of the file and the time it was read. Other parameters can be added, and will be included in the info string on printout.

Addresses #1873

[dlakaplan]

@JPGlaser @rossjjennings : do you think this will do what you need?

[JPGlaser]

This looks great and should work perfectly for our cases.

~ Joe G.

[dlakaplan]

OK, great. Maybe @abhisrkckl or someone else can review & merge.