Allow integer values where doubles are expected in NEST

[heplesser]

Is your feature request related to a problem? Please describe.

NEST does not convert integers to doubles even in cases where this would be unproblematic. The only exception is the Simulate call. Furthermore, the error message emitted if users use integers where double is required is often difficult to comprehend because it does not indicate which entry in a dictionary causes the error, as in this example from the NEST Mailing list:

 File "test/binary_gdf.py", line 9, in <module>
    kk=tp.CreateLayer({'extent': [1, 1],'rows': 20, 'columns': 20,'elements' : 'izhikevich','edge_wrap':True})
  File "/home/neurobit/local/nest-py3/lib64/python3.6/site-packages/nest/topology/hl_api.py", line 650, in CreateLayer
    return topology_func('{ CreateLayer } Map', specs)
  File "/home/neurobit/local/nest-py3/lib64/python3.6/site-packages/nest/topology/ll_api.py", line 61, in topology_func
    return nest.ll_api.sli_func(slifunc, *args)
  File "/home/neurobit/local/nest-py3/lib64/python3.6/site-packages/nest/ll_api.py", line 182, in sli_func
    sli_run(slifun)      # SLI support code to execute s on args
  File "/home/neurobit/local/nest-py3/lib64/python3.6/site-packages/nest/ll_api.py", line 132, in catching_sli_run
    raise exceptionCls(commandname, message)
nest.lib.hl_api_exceptions.TypeMismatch: ('TypeMismatch in CreateLayer_D: Expected datatype: doubletype\nProvided datatype: integertype', 'TypeMismatch', <SLILiteral: CreateLayer_D>, ': Expected datatype: doubletype\nProvided datatype: integertype')

where the problem lies with the integer values for extent.

Describe the solution you'd like

1. NEST should silently convert integers to doubles in keeping with rules for most common programming languages.
2. We should consider whether to accept integer-values doubles where integers are expected (e.g., receptor types)
3. When a type error message is emitted while reading a dictionary entry, the name of the entry should be included in the error message.