Fix issue 500 -- pspm_interp1

[teddychao]

Introduction

Users reported an issue when the input data contains NaNs. The issues is caused by the approach of calculating mean and interpolating at pspm_resp_pp, pspm_prepdata, pspm_interp1. The expected behaviour is listed as below:

1. In pspm_interp1, interpolation should be performed if the data contains at least two non-NaN values.
2. Data as the input of pspm_prepdata should be carefully checked to avoid being converted into NaNs due to other calculations, for example calculating the mean.
   The pull request fixes pspm_interp1 needs to consider data without valid values #500.

Results

Changes proposed in this pull request:

• Update pspm_interp1
  • Add data checking for pspm_interp1.
• Update pspm_resp_pp
  • Avoid NaN converting when performing mean-based filtering.