meshesPath & particlesPath: Optional

Makes the base attributes `meshesPath` and `particlesPath` optional
if no meshes or no particles shall be stored. Specifies that the
path they describe MUST exist as soon as they are set. If they
are unset, this MUST be interpreted as "no respective record".

STANDARD.md

@@ -113,16 +113,26 @@ Each file's *root* directory (path `/`) must at leat contain the attributes:
       of the form given by `basePath` (e.g. `/extra_data`). In this
       way, the openPMD parsing tools will not parse this additional data. 
 
+The following attributes are *optional* in each each file's *root* directory
+(path `/`) and indicate if a file contains mesh and/or particle records. It is
+*required* to set them if one wants to store mesh and/or particle records.
+
   - `meshesPath`
     - type: *(string)*
     - description: path *relative* from the `basePath` to the mesh records
     - example: `meshes/`
+    - note: if this attribute is missing, the file is interpreted as if it
+      contains *no mesh records*! If the attribute is set, the group behind
+      it *must* exist!
 
   - `particlesPath`
     - type: *(string)*
     - description: path *relative* from the `basePath` to the groups for each
                    particle species and the records they include
     - example: `particles/`
+    - note: if this attribute is missing, the file is interpreted as if it
+      contains *no particle records*! If the attribute is set, the group behind
+      it *must* exist!
 
 It is *recommended* that each file's *root* directory (path `/`) further
 contains the attributes: