BUG-FIX: add check in _get_component to check if component is in the model

Fixes confusing expection when m.fit_component is called with a
component which is not in the model.
Fix wrong indention in m._get_component
Add test: checking if component is in model when using m.fit_component
DOC: add info about using different matplotlib colormaps for heatmaps
Removed a newline
Merge pull request #394 from magnunor/ENH_check_if_component_is_in_model

Enh check if component is in model
Merge pull request #395 from magnunor/DOC_heatmap_cmap_info

Doc heatmap cmap info
Merge branch 'master' into EHN_warning_line_ouside_limit
EHN: New name _get_xray_lines_in_spectral_range
Test: add test for Xray lower that energy range
EHN: add warning.warn