add attributes “units” and “minIML” and “maxIML” in both fragility models (continuous and discrete)

[Muharem Hrnjadovic]

This requires changes in
  - nrml model
  - database schema
  - nrml parser
  - database writer

[Muharem Hrnjadovic (al-maisan)]

From Vitor's email:

    1. What is the "units" attribute about? Is it comparable to the same
       attribute in the exposure model?
       What is its type and what are the possible values? Are there any
       constraints that should be imposed?

No, units here refer to how the IML are being measured. So common values would be "g", "m", or even none if macro seismic intensity was being used. This being said, this parameter should be a string with no constrains and not mandatory.

    2. What is the type of the "minIML" attribute and what are the possible
       values? Are there any constraints that should be imposed?

    3. What is the type of the "maxIML" attribute and what are the possible
       values? Are there any constraints that should be imposed?

These parameters are purely informative and will not be used in the calculations for the time being. Therefore they are optional and defined as a double.

[Muharem Hrnjadovic (al-maisan)]

Another exchange with Vitor yielded the following clarification:

These new properties apply to the entire fragility model.

In case of a discrete model we only allow the "units" property since the IMLs are already there.

In the continuous fragility model case all 3 properties (i.e. "units", and "minIML" and "maxIML") are allowed .