Base paths for mesh- and particle-only files and updated attributes

This release defines how to handle files that only contain mesh or
particle records gracefully. New attributes have been defined to
document software dependencies and utilized hardware. The ED-PIC
extension updated values for particle pushers. It further improves
wordings for consistency throughout the standard.

Changes to "1.0.1"
------------------

Base Standard
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- **minor changes:**
  - `meshesPath` & `particlesPath`: optional (only when needed) #171
  - optional attributes to document software dependencies & hardware #170
    - `softwareDependencies`
    - `machine`
- **backwards compatible changes:**
  - fix wording: `meshName` means `mesh record` #168
  - fix wording: root path is a "group" as well #169

Extension
"""""""""

- `ED-PIC`:
  - **minor changes:**
    - `particlePush`: additional values defined #172

Data Converter
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No data conversion needed.

Typo and Wording Changes

This release improves wordings, common questions and example snippets.
Keywords and logic are unchanged and backwards compatible for revision releases.

Changes to "1.0.0"
------------------

Base Standard
"""""""""""""

- **backwards compatible changes:**
  - better examples for `axisLabels` #153
  - unspecified (user-chosen) `float`/`uint`/`int` are now called `floatX`/`uintX`/`intX` throughout the standard #152
  - note to store non-openPMD information outside the `basePath` #115
  - particle position: update python example #119 #127
  - particle patches: typo in reference to `positionOffset` #135

Extension
"""""""""

- `ED-PIC`:
  - **backwards compatible changes:**
    - `fieldSmoothing`: typo in allowed values #131
    - `fieldSolver`: typo in PSATD description #133
    - reference updated to unspecified (user-chosen) `float`/`uint`/`int` as in base standard #152

Data Converter
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No data conversion needed.

Update Date of Release, Minor Wordings

Replace "user" with "implementor" and fix a typo in the
python example for positions.