Rephrase "exactly one" to "at least one" selection groups

Author: leonlan

README.md

@@ -53,7 +53,7 @@ To parse a specific instance format, set the `instance_format` argument in `pars
 Specifically, we included an extra line that defines for each task whether it is optional or not.
 6. `aslib`: The **ASLIB format** is the format used by RCPSP-AS instances from the ASLIB instance set at [OR&S project database](https://www.projectmanagement.ugent.be/research/data).
 ASLIB consist of three different parts (a, b, c). 
-To use this parser, you have to merge parts (a) and (b) into a single file - part (c) is not parsed.
+To use this parser, you have to merge parts (a) and (b) into a single file - part (c) is not parsed. See [this](https://gist.github.com/leonlan/1064a9e785d4dca78b0839984edb1691) gist.
 7. `mslib`: The **MSLIB format** is the format used by MSRCPSP instances from the MSLIB instance set at [OR&S project database](https://www.projectmanagement.ugent.be/research/project_scheduling/MSRCPSP).
 
 ## Instance databases

example.ipynb

@@ -347,7 +347,7 @@
     "\n",
     "1. Activities can be optional - they don't all need to be scheduled\n",
     "2. Activities each have a set of associated selection groups\n",
-    "3. If an activity is scheduled, then for each selection group, exactly one activity must also be scheduled"
+    "3. If an activity is scheduled, then for each selection group, at least one activity must also be scheduled"
    ]
   },
   {

psplib/ProjectInstance.py

@@ -64,7 +64,7 @@ class Activity:
         Whether this activity is optional or not. Default ``False``.
     selection_groups
         The selection groups of this activity. If the current activity is
-        scheduled, then for each group, exactly one activity must be scheduled.
+        scheduled, then for each group at least one activity must be scheduled.
         This is used for RCPSP-PS instances. Default is an empty list.
     name
         Optional name of the activity to identify this activity. This is