Ensure that results from repeated runs are deterministic

Here's the output of a simple script:


```
from astrodendro import Dendrogram
from astropy.io import fits
image = fits.getdata('L1448_13CO.fits')
d = Dendrogram.compute(image, min_value=1.2, min_delta=0.2, min_npix=10, verbose=True)
print(list(d.all_structures)[0].indices())
```

The output is not always the same which is going to cause issues if users try and reproduce bugs. Also, structure IDs may not always refer to the same structures, which would be an issue. There are several places where dictionaries are used, so this must be what is causing the differences:

```
mac-robitaille2:dendro-core tom$ python test.py 
Generating dendrogram using 78,465 of 6,637,050 pixels (1.1822270436413769% of data)
[========================================>] 100%
(array([223, 222, 224, 217, 221, 217, 217, 221, 222, 222, 221, 219, 223,
       220, 223, 219, 222, 220, 220, 220, 223, 221, 222, 218, 217, 222,
       222, 220, 222, 220, 221, 219, 221, 221, 221, 221, 220, 221, 220,
       224, 219, 221, 221, 221, 222, 222, 220, 222, 220, 218, 218, 219,
       219, 218, 221, 221, 223, 223, 221, 219, 220, 224, 224, 225, 221,
       223, 223, 219, 221, 220, 221, 218, 220, 223, 224, 216, 219, 220, 216]), array([44, 44, 45, 44, 42, 44, 44, 42, 46, 43, 42, 43, 44, 45, 44, 43, 45,
       43, 43, 43, 44, 45, 44, 43, 43, 45, 45, 44, 45, 44, 43, 43, 43, 43,
       43, 43, 44, 43, 44, 45, 43, 45, 45, 45, 44, 44, 45, 45, 43, 44, 44,
       44, 44, 44, 44, 46, 45, 45, 44, 44, 42, 44, 44, 46, 44, 45, 43, 44,
       44, 42, 44, 44, 43, 46, 46, 43, 45, 43, 43]), array([81, 79, 80, 78, 85, 79, 80, 84, 80, 81, 83, 81, 80, 79, 79, 80, 78,
       84, 82, 81, 82, 82, 81, 82, 79, 80, 81, 81, 82, 80, 84, 83, 83, 82,
       81, 80, 79, 79, 78, 79, 82, 78, 79, 81, 82, 80, 81, 79, 80, 81, 80,
       81, 82, 79, 80, 79, 79, 80, 79, 79, 82, 79, 81, 79, 81, 81, 82, 80,
       82, 84, 78, 78, 79, 80, 79, 78, 79, 78, 79]))
mac-robitaille2:dendro-core tom$ python test.py 
Generating dendrogram using 78,465 of 6,637,050 pixels (1.1822270436413769% of data)
[========================================>] 100%
(array([156, 156, 154, 156, 156, 154, 156, 154, 154, 156, 155, 153, 154,
       155, 154, 156, 155, 154, 153, 157, 158, 158, 154, 155, 153, 154,
       156, 155, 155, 154, 156, 154, 153, 156, 153, 154, 153, 154, 155,
       155, 155, 155, 153, 155, 153, 155, 155, 155, 156, 153, 158, 158,
       155, 155, 155, 155, 153, 155, 156, 156, 157, 154, 157, 154, 154,
       154, 155, 155, 155, 155, 155, 154, 154, 154, 153, 152, 155, 154,
       155, 154, 155, 158, 158, 156, 156, 155, 157, 157, 156, 155, 157,
       157, 156, 155, 154, 155, 155, 154, 154, 155, 158, 154]), array([75, 78, 78, 78, 73, 77, 75, 80, 77, 75, 78, 73, 78, 76, 73, 73, 81,
       77, 79, 74, 75, 76, 77, 76, 76, 78, 74, 78, 81, 76, 80, 78, 78, 74,
       79, 78, 78, 77, 78, 77, 77, 72, 77, 72, 76, 77, 78, 78, 81, 76, 76,
       76, 76, 75, 74, 79, 77, 79, 76, 79, 75, 76, 73, 79, 79, 79, 75, 75,
       75, 79, 75, 79, 79, 80, 77, 78, 80, 74, 73, 79, 80, 76, 76, 76, 77,
       73, 76, 76, 77, 73, 78, 76, 77, 74, 75, 74, 76, 75, 75, 76, 75, 78]), array([37, 34, 37, 33, 37, 36, 36, 34, 35, 34, 32, 37, 33, 34, 37, 38, 35,
       33, 35, 37, 37, 37, 34, 39, 35, 39, 37, 35, 34, 37, 34, 35, 36, 39,
       33, 36, 35, 39, 36, 34, 35, 39, 35, 40, 37, 39, 34, 33, 34, 36, 38,
       39, 38, 34, 36, 36, 34, 37, 37, 33, 37, 35, 37, 34, 35, 36, 38, 37,
       36, 38, 35, 37, 38, 35, 36, 36, 34, 37, 39, 33, 35, 40, 41, 36, 33,
       36, 37, 38, 34, 38, 33, 41, 35, 39, 38, 37, 36, 37, 36, 33, 41, 34]))
```