cpu - fix element/block evec lengths

vec/qf - initial valid/borrowed/owned split for data (#853)

* vec/qf - initial valid/borrowed/owned split for data

* vec/qf - tidy logic for checking active/stale data

* minor - add missing NU

vec/qf - initial valid/borrowed/owned split for data (#853)

* vec/qf - initial valid/borrowed/owned split for data

* vec/qf - tidy logic for checking active/stale data

* minor - add missing NULL

* doc - explain VectorTakeArray update

* minor - update error messages

* test - update error message in junit/tap

* gpu - fix stray CeedScalar vs void for QFunctionContext

* vec/qf - clarify/simplify access logic

* vec - calloc host arrays when no value set to make empty

* style - minor

* style - minor

* minor - fix error messages

* vec/qf - move data validity checking to backend interface

* gpu - add missing sync error checking for qfcontext

* gpu - homogonize use of impl for backend data to reduce confusion

* vec - clarify access conditions

* python - update test for stricter vector access

* vec - minor fixes

* minor - fix ipython change

* vec - add missing declarations in ceed/backend.h

* ctx - mirror vector borrowed data check in ctx interface

* vec - add CeedVectorGetArrayWrite

* vec - consistent use of CeedVectorGetArray vs CeedVectorGetArrayWrite

* python - small vec fixes

* doc - describe vector data semantics

* magma - update restriction

* gpu - fix restr bug I added, need to sum into target

* magma - fix restriction bug

* cpu - fix restriction bug here too

* op - fix evec allocations

* julia - fix ElemRestriction for new vector access rules

* op - double check GetArray vs Read vs Write usage

* doc - small fix

* restr - clean up read/write logic for restr

* python - add vec.array_write

* magma - typo fix

show more ...

gpu - remove edata from backend operator structs

cpu - additional minor name updates

cpu - fix unclear variable names in operator impl

cpu - remove e_data array from backend operator data

op - turn magic number 16 into constant CEED_FIELD_MAX

ceed - split CeedRegister into two parts for easier debugging output

debug - list all backends registered

minor - remove template backend/qfunction from registration

debug - create CeedDebugEnv macro, refactor CeedDebug macro

xsmm - fix minor inconsistency in filename

Merge pull request #841 from CEED/jeremy/qf-multi-source

QFunction multi source support

ref - add missing static

opt - specalize tensor product for C=1 case

tidy - suppress spurious warning

gpu - refactor common jit code into common functions

qf - add function to retreive kernel name, stored path now excludes kernel name

occa - add warning, OCCA backend is experimental

qf - cache more QF assembly components for CPU

gpu - cache two more vectors used in operator diagonal assembly

tidy - minor fixes in CUDA

op - add CeedOperatorLinearAssembleQFunctionBuildOrUpdate to reduce kernel calls in diagonal/full/fdm assembly

cuda - drop unused function declarations

interface - refactor *GetFields to include number of fields