Frequently (e.g., fields of internal varialbles for material laws), a field is
used through only one map and this map never changes. The procedure of
registering a field and mapping it in the constructor of materials is
unnecessarily cumbersome here. Mapped fields fix this issue by encapsulating a
field (data storage) and its map (data access) into a single object.

commit a2f265dc
Author: Till Junge <till.junge@epfl.ch>
Date:   Thu Apr 25 11:33:35 2019 +0200

    DOC: ready for discussion

commit 56b0fcb2
Author: Till Junge <till.junge@epfl.ch>
Date:   Thu Apr 18 15:26:43 2019 +0200

    DOC: work in progress

commit 8f01d6ba
Merge: 03648b1f 62479808
Author: Till Junge <till.junge@epfl.ch>
Date:   Thu Apr 4 09:17:18 2019 +0200

    Merge branch 'feat/17-rethinking-material-laws' of gitlab.com:muspectre/muspectre into feat/17-rethinking-material-laws

commit 03648b1f
Author: Till Junge <till.junge@epfl.ch>
Date:   Wed Apr 3 17:36:13 2019 +0200

    WIP: working on field documentation

commit 62479808
Author: Till Junge <till.junge@epfl.ch>
Date:   Mon Apr 1 08:28:34 2019 +0200

    DOC: work in progress

commit d0e75e0f
Author: Till Junge <till.junge@epfl.ch>
Date:   Fri Mar 29 16:59:55 2019 +0100

    WIP: working on field documentation

commit 0cff781b
Author: Till Junge <till.junge@epfl.ch>
Date:   Thu Mar 28 17:07:06 2019 +0100

    WIP: Work-around for new g++-8 internal compiler error

commit cbc0fc6a
Author: Till Junge <till.junge@epfl.ch>
Date:   Thu Mar 28 15:24:42 2019 +0100

    WIP: started documentation of µGrid

commit 249073a7
Author: Till Junge <till.junge@epfl.ch>
Date:   Thu Mar 28 11:03:03 2019 +0100

    MAINT: Switched MaterialHyperElastoPlastic1 over MappedFields

commit 26482008
Author: Till Junge <till.junge@epfl.ch>
Date:   Thu Mar 28 09:37:22 2019 +0100

    WIP: Linted additions to test_goodies

commit 2ae60b0b
Author: Till Junge <till.junge@epfl.ch>
Date:   Wed Mar 27 22:42:15 2019 +0100

    WIP: added random access operator to MappedField

commit 60f299e6
Author: Till Junge <till.junge@epfl.ch>
Date:   Wed Mar 27 19:45:04 2019 +0100

    MAINT: linting

commit 62fddf9a
Author: Till Junge <till.junge@epfl.ch>
Date:   Wed Mar 27 19:40:57 2019 +0100

    WIP: Fixing MappedFields

commit de7d37c2
Author: Till Junge <till.junge@epfl.ch>
Date:   Wed Mar 27 19:33:59 2019 +0100

    ENH: Introduced MappedFields