Improvements on API documentation

[ojeda-e]

I have a couple of suggestions to improve the readability of the docs.

1. API geometry.

• The order parameter equation is displayed as text. Suggestion: Using the .. math:: sphinx markup for equations will make it more readable.
• The diagram in docstrings, from lines 184-188, doesn't display properly. Given the aim of the package, displaying it properly is relevant. Adding backquotes may help to fix it.

2. API hydrogens

• In docstrings, all the return types are plain text. In general, backquotes are used for inline code.
  Example: For lines 29-32 in hydrogens.py
  Suggested:

    Returns
    -------
    numpy 1D-array
        Coordinates of the rebuilt hydrogen: ``([x_H, y_H, z_H])``.

3. API init_dics

• As in API hydrogens, use backquotes (``) or (```) for inline code.
  For example, lines 54-58 should be inline code.

4. API lipids and utils

• In buildh.lipids.check_topology, the type of dict is dict but in previous API pages appears as dictionary. For consistency, it would be better to have one only. Same for the types of parameters in buildh.utils.is_allHs_present and buildh.utils.check_slice_options (dict vs dictionary).

[patrickfuchs]

Thanks for the useful suggestions @ojeda-e. @HubLot will implement all of these within a couple of days.

[HubLot]

Thanks @ojeda-e for your detailed issue. I create a PR (#154) which should fix all the points raised and some others I spotted when reviewing the docstrings.