Update pbr.hpp for v3 (libsemigroups#600)

* Bump copyright

* identity->one

* make -> to_pbr

* Create pbr namespace

* Add _no_checks suffix

* Update docs

* Remove bad inline

* Update validation

* Remove duplication

* Fix bad _no_checks

* Use PBR defined types

* size -> number_of_points

* Make one a free-function

* Update doc wording

docs/DoxygenLayout.xml

@@ -41,6 +41,7 @@
       <tab type="user" visible="yes" url="@ref adapters_matrix_group"
         title="Adapters for other matrices" />
       <tab type="user" visible="yes" url="@ref adapters_hpcombi_group" title="Adapters for HPCombi" />
+      <tab type="user" visible="yes" url="@ref adapters_pbr_group" title="Adapters for PBR" />
     </tab>
     <tab type="usergroup" visible="yes" url="@ref elements_group" title="Elements">
       <tab type="user" visible="yes" url="@ref bipart_group" title="Bipartitions" />
@@ -74,6 +75,8 @@
             title="StaticRowView" />
         </tab>
       </tab>
+      <tab type="user" visible="yes" url="@ref pbr_group"
+        title="Partitioned binary relations (PBRs)" />
       <tab type="user" visible="yes" url="@ref transf_group" title="Transformations" />
     </tab>
     <tab type="user" visible="yes" url="@ref orders_group" title="Orders" />

docs/libsemigroups.bib

@@ -277,6 +277,16 @@ @article{Maltcev2007aa
 	year = 2007,
 	bdsk-url-1 = {https://doi.org/10.21136/mb.2007.134125}}
 
+@misc{Martin2011aa,
+      title={Partitioned binary relations},
+      author={Paul Martin and Volodymyr Mazorchuk},
+      year={2011},
+      eprint={1102.0862},
+      archivePrefix={arXiv},
+      primaryClass={math.RT},
+      url={https://arxiv.org/abs/1102.0862},
+}
+
 @article{Mitchell2021aa,
 	author = {J. D. Mitchell and M. Tsalakou},
 	date-added = {2021-03-09 15:23:03 +0000},

include/libsemigroups/bipart.hpp

@@ -778,7 +778,7 @@ namespace libsemigroups {
   //! used in the GAP package [Semigroups package for
   //! GAP](https://semigroups.github.io/Semigroups/).
   //!
-  //! \sa libsemigroups::validate(Bipartition const&).
+  //! \sa bipartition::validate(Bipartition const&).
   // TODO(2) add more explanation to the doc here
   class Bipartition {
    private: