YAML with a colon but no space is valid YAML, but produces a string instead of a map

## The problem

In httomo pipeline files mappings are used fairly frequently, such as when defining the parameters and parameter values for a method:
```yaml
- method: remove_stripe_based_sorting
module_path: httomolibgpu.prep.stripe
parameters:
size: 11
dim: 1
```

From some unexpected behaviour when running httomo at a beamline, investigations have led to the discovery that a mapping in YAML must have either:
- at least one space after the colon character (if defining a key-value pair whose value is a non-collection type, such as an int)
```yaml
center: 20.5
```
- a newline character (if defining a key-value pair whose value is a collection type of some sort, such as a list)
```yaml
parameters:
- item_one
- ...
```

For the first case, if there is no space after the colon character, then the text following is parsed as a python `str`, rather than as a python `dict`.

This means that if a user were to miss a space after a colon character, then this can cause the YAML to be parsed differently to how it was intended, and thus cause runtime errors.

## Explanation

For example, for the following YAML (note the spaces after the colon characters after `start` and `stop`):
```yaml
- method: standard_tomo
module_path: httomo.data.hdf.loaders
parameters:
data_path: entry1/tomo_entry/data/data
image_key_path: entry1/tomo_entry/instrument/detector/image_key
rotation_angles:
data_path: /entry1/tomo_entry/data/rotation_angle
preview:
detector_y:
start: 100
stop: 120
```

the `detector_y` field and its value parses to the following python data structure (note that the value of the `detector_y` key in the dict is a `dict`):
```python
{
'detector_y': {'start': 100, 'stop': 120}
}
```

However, if the spaces after the two colon characters are omitted (the change in syntax highlighting compared to the previous example is suggestive of there being a change in meaning of the value):
```yaml
- method: standard_tomo
module_path: httomo.data.hdf.loaders
parameters:
data_path: entry1/tomo_entry/data/data
image_key_path: entry1/tomo_entry/instrument/detector/image_key
rotation_angles:
data_path: /entry1/tomo_entry/data/rotation_angle
preview:
detector_y:
start:100
stop:120
```

the `detector_y` field and its value parses to the following python data structure (notice that the value of the `detector_y` key in the dict is a _string_):
```python
{
'detector_y': 'start:100 stop:120'
}
```

I haven't found anything that explicitly states that without a space after a colon, the value is interpreted as a string. The closest I can find is in the YAML spec [here](https://yaml.org/spec/1.2.2/#62-separation-spaces) where it states that a whitespace character needs to follow a colon in order to define a mapping (but it doesn't say what happens if you omit the colon, it may be somewhere else in the YAML spec, but it's a long document to sift through...).

## What can be done?

I'm not sure yet. I took at a look to see if any YAML linters would allow the catching of if a colon was missing a space after it, such as `yamllint`, which has some options for configuring the rules when encountering a colon character.

However, it seems like it's not possible to catch this (partly due to it being valid YAML to have a missing space after a colon), see https://github.com/adrienverge/yamllint/issues/563#issuecomment-1508763231 and the rest of that issue.

In particular, in that comment, it's suggested that conversion to JSON, followed by defining a JSON schema, could solve the problem.

With the development of the web GUI moving forward and it involving this very idea of using a JSON schema to perform validation of parameters in pipeline files, this may well be an issue that could be resolved in that larger discussion.