Fix/unresr

[whaeck]

Additional messages in UNRESR for signaling issue #116.

The changes are as follows:

• a message is issued when the background plus the potential scattering, the interference and sigma0 value are negative
• a message is issued when the beta value is within the -1 to 0 range (which causes a sqrt to return NaN)

Questions:

• the first message could be slightly changed so that it is issued when the background plus the potential scattering and the interference are negative, regardless of the value of sigma0. In my opinion, the fact that this value is negative before adding sigma0 is an indication that the evaluation is no good whatever happens.
• the second message could be transformed into an error and that would stop the processing dead in its tracks (which degrades the behaviour from before these changes - it was silently ignored). In my opinion, the results will be inherently wrong, which would merit the use of error instead of mess.

[coveralls]

Coverage remained the same at ?% when pulling d57f787827dc29ae99276e46ef0c10a72dfb1377 on fix/unresr into f03b64494277a01e561dee84e96d4b570f417312 on master.